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RAP , Right anterior See also:

process. tendon. PhN, Pharyngeal notch. PLP, Posterior lateral process. ALR, Anterior lateral See also:rod or tendon. Natural See also:size. (From Lankester, Q. J. Mic. Sci., N.S. vol. See also:xxiv., 1884.) Limulus an Arachnid.—See also:Modern views as to the See also:classification and See also:affinities of the See also:Arachnida have been determined by the demonstration that Limulus and the See also:extinct Eurypterines (Pterygolus, &c.) are Arachnida; that is to say, are identical in the structure and relation of so many important parts with See also:Scorpio, whilst differing in those respects from other See also:Arthropoda, that it is impossible to suppose that the identity is due to homoplasy or convergence, and the conclusion must be accepted that the resemblances arise from See also:close genetic relationship. The view that Limulus, the See also:king-crab, is an Arachnid was maintained as See also:long ago as 1829 by See also:Strauss-Durckheim (1), on the ground of its See also:possession of an See also:internal cartilaginous sternum—also possessed by the Arachnida (see See also:figs. 1, 2, 3, 4, 5 and 6)—and of the similarity of the disposition of the six See also:leg-like appendages around the mouth in the two cases (see figs. 45 and 63).

The See also:

evidence of the exact equivalence of the segmentation and appendages of Limulus and Scorpio, and of a number of remarkable points of agreement in structure, was furnished by See also:Ray Lankester in an See also:article published in 1881 (" Limulus an Arachnid," Quart. Journ. Micr. Sci. vol. xxi. N.S.), and in a See also:series of subsequent See also:memoirs, in which the structure of the entosternum, of the coxal glands, of the eyes, of ' the veno-pericardiaie muscles, of the See also:respiratory lamellae, and of other parts, was for the first See also:time described, and in which the new facts discovered were shown uniformly to support the See also:hypothesis that Limulus is an Arachnid. A See also:list of these memoirs is given at the close of this article (2, 3, 4, 5 and 13). The Eurypterines (Gigantostraca) were included in the See also:identification, although at that time they were supposed rftrR (From Lankester.) to possess only five pairs of anterior or prosomatic appendages. They have now been shown to possess six pairs (fig. 47), as do Limulus and Scorpio. The various comparisons previously made between the struc- See also:ture of Limulus and the Eurypterines on the one See also:hand, and that of a typical Arachnid, such as Scorpio, on the other, had been vitiated by erroneous notions as to the origin of the nerves supplying the anterior appendages of Limulus (which were finally removed by See also:Alphonse Milne-See also:Edwards in his beautiful memoir (6) on the structure of that See also:animal), and secondly by the errone- ous identification of the See also:double sternal plates of Limulus, called " chilaria," by See also:Owen, with a pair of append- ages (7). Once the identity of the chilaria with the pentagonal sternal See also:plate of the See also:scorpion is recognized —an identification first insisted on by Lan- kester—the whole series of segments and appendages in the two animals, Limulus and Scorpio, are seen to See also:cor- See also:respond most closely, segment for segment, with one another (see figs. 7 and 8).

The structure of the proso- matic appendages or legs is also seen to pre- sent many significant points of agreement (see figures), but a curi- ous discrepancy existed (After Lankester, toe. cu .) in the six-jointed struc- ture of the See also:

limb in Limulus, which differed from the seven-jointed limb of Scorpio by the defect of one See also:joint. R. I. See also:Pocock of the See also:British Museum has observed that in Limulus a marking exists on the See also:fourth joint, which apparently indicates a previous See also:division of this segment into two, and thus establishes the agreement of Limulus and Scorpio in this small feature of the number of segments in the legs (see fig. 11). It is not desirable to occupy the limited space of this article by a full description of the limbs and segments of Limulus and Scorpio. The reader is referred to the See also:complete series of figures here given, with their explanatory legends (figs. 12, 13, 14, 15). Certain matters, however, require comment and explanation to render the comparison intelligible. The tergites, or chitinized dorsal halves of the See also:body rings, are fused to See also:form a " prosomatic See also:carapace," or carapace of the prosoma, in both Limulus and Scorpio (see figs. 7 and 8). This region corresponds in both cases to six somites, as .indi- cated by the presence of six Zmjr.. pairs of limbs.

On the sur- See also:

face of the carapace there are , PP--in both animals a pair of "'"-- central eyes with See also:simple See also:lens rf >' and a pair of lateral See also:eye-tracts, which in Limulus consist of closely-aggregated simple eyes, forming a " See also:compound" eye, whilst in Scorpio they See also:present several See also:separate small eyes. The microscopic structure of the FIG. 4.-Ventral See also:surface of the same central and the lateral eyes entosternum as that See also:drawn in fig. 3. has been shown by Lankester Letters as in fig. 3 with the addition and A. G. See also:Bourne (5) to of NC, neural See also:canal or foramen. differ; but the lateral eyes of Scorpio were shown by them (After Lankester, loc. cit.) to be similar in structure to the lateral eyes of Limulus, and the central eyes of Scorpio to be identical in structure with the central eyes of Limulus (see below). Following the prosoma is a region consisting of six segments (figs. 14 and 15), each carrying a pair of plate-like appendages in both Limulus and Scorpio. This region is called the mesosoma. The tergites of this region and those of the following region, the metasoma, are fused to form a second or posterior carapace in Limulus, whilst remaining See also:free in Scorpio.

The first pair of foliaceous appendages in each animal is the genital operculum; beneath it are found the openings of the genital ducts. The second pair of mesosomatic append-ages in Scorpio are known as the " pectens." Each consists of an See also:

axis, bearing numerous See also:blunt tooth-like See also:pro- FIG. 5.-Entosternum of cesses arranged in a series. This is one of the mygalomorphous represented in Limulus by the first gill- See also:spiders; ventral surface. bearing appendage. The leaves (some Ph.N., pharyngeal notch. 150 in number) of the gill-See also:book (see The posterior median pro-figure) correspond to the tooth-like See also:cess with its repetition of processes of the pectens of Scorpio. triangular segments closely The next four pairs of appendages (See also:corn- resembles the same process pleting the mesosomatic series of six) in Limulus, consist, in both Scorpio and Limulus, of a See also:base carrying each 130 to 150 (From Lankester, loc. cit.) See also:blood-holding, See also:leaf-like plates, lying on one another like the leaves of a book. Their See also:minute structure is closely similar in the two cases; the leaf-like plates receive blood from the See also:great sternal sinus, and serve as respiratory See also:organs. The difference between the gill-books of Limulus and the See also:lung-books of Scorpio depends on the fact that the latter are adapted to aerial respiration, while the former serve for aquatic respiration. The appendage carrying the gill-book stands out on the surface of the body in Limulus, and has other portions See also:developed besides the gill-book and its base; it is fused with its See also:fellow of the opposite See also:side On the other hand, in Scorpio, the gill-book-bearing ap- FIG. 6.-Dorsal surface of ' Forming has sunk below the surface, forming a See also:recess or chamber for the same e.ntosternum as itself, which communicates with the that drawn in fig. 5. Ph.N., exterior by an See also:oval or circular pharyngeal notch.

" stigma " (fig. lo, stg). That this (Alter Lankester, loc. cit.) in-sinking has taken See also:

place, and that the lung-books or in-sunken gill-books of Scorpio really represent appendages (that is to say, limbs or parapodia) is proved by their developmental See also:history (see AC. m=. m=. snp, Sub-neural See also:arch. ap, Anterior lateral process (same as RAP and See also:LAP in fig. I). See also:Imp, Lateral median process (same as ALR and PLR of fig. I). pp, Posterior process (same as PLP in fig. I). pf, Posterior flap or See also:diaphragm of New-See also:port. ml and m2, Perforations of the diaphragm for the passage of muscles. DR, The paired dorsal ridges.

GC, Gastric canal or foramen. AC, Arterial canal or foramen. figs. 17 and 18). They appear at first as outstanding processes on the surface of the body. The exact mode in which the in-sinking of superficial outstanding limbs, carrying gill-lamellae, has historically taken place has been a See also:

matter of much See also:speculation. It was to be hoped that the specimen of the See also:Silurian scorpion (Palaeophonus) from See also:Scotland, showing the ventral surface of the mesosoma (fig. 49), would throw See also:light on this matter; but the specimen recently carefully studied by the writer and Pocock reveals neither gill-bearing limbs nor stigmata. The See also:probability appears to be against an actual introversion of the appendage and its lamellae, as was at one time suggested by Lankester. It is probable that such an in-sinking as is shown in the oc oc, Lateral compound eyes. See also:wise suppressed praegenital oc', Central monomeniscous eyes. somite. PA, See also:Post-anal spine. VIII to XIII, The six somites of I to VI, The six appendage- the mesosoma, each with a bearing somites of the pro- movable pleural spine and a See also:soma. pair of dorsal entopophysis or VII, Usually considered to be muscle-attaching ingrowths. the tergum of the genital XIV to XVIII, The confluent or somite, but suggested by unexpressed six somites of the Pocock to be that of the other- metasoma.

[According to the See also:

system of numbering explained in the See also:text, if VII is the tergum of the praegenital somite (as is probable) it should be labelled Prg without any number, and the somites VIII to XIII should be lettered i to 6, indicating that they are the six normal somites of the mesosoma; whilst XV to XVIII should be replaced by the See also:numbers 7 to 12—an additional suppressed segment (making up the typical six) being reckoned to the metasomatic See also:fusion.] (From Lankester, Q. J. Mier. Sci. vol. xxi., is8r.) accompanying See also:diagram has taken place (fig. 15) ; but we are yet in need of evidence as to the exact equivalence of margins, axis, &c., obtaining between the lung-book of Scorpio and the gill-book of Limulus. Zoologists are See also:familiar with many instances (fishes, crustaceans) in which the protective walls of a See also:water-breathing See also:organ or gill-apparatus become converted into an See also:air-breathing organ or lung, but there is no other See also:case known of the See also:conversion of gill processes themselves into air-breathing plates. The Identification of the lung-books of Scorpio with the gill-books of Limulus is practically settled by the existence of the pectens in Scorpio (fig. 14, VIII) on the second mesosomatic somite. There is no doubt that these are parapodial or limb appendages, carrying numerous imbricated secondary processes, and therefore comparable in essential structure to the leaf-bearing plates of the second meso- IL I0somatic somite of Limulus. They have remained unenclosed and projecting on the surface of the body, as once were the appendages of the four following somites. But they have lost their respiratory See also:function. In non-aquatic See also:life such an unprotected organ cannot subserve respiration.

The " pectens " have become more firmly chitinized and probably somewhat altered in shape as compared with their See also:

condition in the aquatic ancestral scorpions. Their present function in scorpions is not ascertained. They are not specially sensitive under See also:ordinary conditions, and may be touched or even pinched without causing any discomfort to the scorpion. It is probable that they acquire See also:special sensibility at the breeding See also:season and serve as " guides " in copulation. The shape of the legs and the See also:absence of paired terminal claws in the Silurian Palaeophonus (see figs. 48 and 49) as compared with living scorpions (see fig. lo) show that the See also:early oc... scorpions were aquatic, and we may See also:hope some See also:day in better-preserved specimens than the two as yet discovered, to find the respira-0ctory organs of those creatures in the condition of projecting appendages serving aquatic respiration somewhat as in Limulus, though not necessarily repeating the exact form of the broad plates of Limulus. It is important to See also:note that the series of lamellae of the lung-book and the gill-book correspond exactly in structure, the narrow, See also:flat blood-space in the lamellae being interrupted by See also:pillar-like junctions of the two surfaces in both cases (see Lankester (4)), and the free surfaces of the adjacent lamellae being covered with a very delicate chitinous cuticle which is drawn out into delicate hairs and processes. The elongated axis which opens at the stigma in Scorpio and which can be cleared of soft, surrounding tissues and coagulated blood so as to present the See also:appearance of a limb axis carrying the book-like leaves of the lung is not really, as it would seem to be at first sight, the limb axis. That is necessarily a blood-holding structure and is obliterated and fused with soft tissues of the sternal region so that the lamellae cannot be detached and presented as See also:standing out from it. The apparent axis or basal support of the scorpion's lung-books shown in the figures, is a false or secondary axis and merely a See also:part of the infolded surface which forms the air-chamber. The maceration of the soft parts of a scorpion preserved in weak spirit and the cleaning of the chitinized in-grown cuticle give rise to the false appearance of a limb axis carrying the lamellae. The margins of the lamellae of the scorpion's lung-book; which are lowermost in the figures (fig.

15) and appear to be free, are really those which are attached to the blood-holding axis. The true free ends are those nearest the stigma. Passing on now from the mesosoma we come in Scorpio to the metasoma of six segments, the first of which is broad whilst the See also:

rest are cylindrical. The last is perforated by the anus and carries the post-anal spine or sting. The somites of the metasoma carry no parapodia. In Limulus the metasoma is practically suppressed. In the allied extinct Eurypterines it is well developed, and resembles that of Scorpio. In the embryo Limulus (fig. 42) the six somites of the mesosoma are not fused to form a carapace at an early See also:stage, and they are followed by three separately marked metasomatic somites; the other three somites of the metasoma have disappeared in Limulus, but are. represented by the unsegmented prae-anal region. It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites. It would be possible to suppose, on the other hand, that new somites are only beginning to make their appearance here. The See also:balance of various considerations is against the latter hypothesis.

Following the metasoma in Limulus, we have as in Scorpio the post-anal spine in this case not a sting, but a powerful and important organ of locomotion, serving to turn the animal over when it has fallen upon its back. The nature of the post-anal spine has been strangely misinterpreted by some writers. Owen (7) maintained that it represented a number of coalesced somites, regardless of its post-anal position and mode of development. The agreement of the grouping of the somites, of the form of the parapodia (appendages, limbs) in each region, of the position of the genital See also:

aperture and operculum, of the position and See also:character of the eyes, and of the powerful post-anal spines not seen in other Arthropods, is very convincing as to the See also:affinity LI (From Lankester, See also:lac. cit.) of Limulus and Scorpio. Perhaps the most important See also:general agreement of Scorpio compared with Limulus and the Eurypterines is the division of the body into the three regions (or tagmata)—prosoma, mesosoma and metasoma—each consisting of six segments, the prosoma having leg-like appendages, the mesosoma having foliaceous appendages, and the metasoma being destitute of appendages. In 1893, some years after the identification of the somites of Limulus with those of Scorpio, thus indicated, had been published, zoologists were startled by the See also:discovery by a See also:Japanese zoologist, Kishinouye (8), of a seventh prosomatic somite in the embryo of Limulus longispina. This was seen in See also:longitudinal sections, as shown in fig. 19. The simple identification of somite with somite in Limulus and Scorpio seemed to be threatened by this discovery. But in 1896 Dr See also:August Brauer of See also:Marburg (9) discovered in the embryo of Scorpio a seventh prosomatic somite (see VII PrG, figs. 17 and 18), or, if we please so to See also:term it, a praegenital somite, hitherto unrecognized. In the case of Scorpio this segment is indicated in the embryo by the presence of a pair of rudimentary appendages, carried by a well-marked somite.

As in Limulus, so in Scorpio, this unexpected somite and its appendages disappear in the course of development. In fact, more or less complete " excalation " of the somite takes place. Owing to its position it is convenient to term the somite which is excalated in Limulus and Scorpio " the praegenital somite." It appears not improbable that the sternal plates wedged in between (After Lankester, loc. cit.). the last pair of legs in both Scorpio and Limulus, viz. the pentagonal sternite of Scorpio (fig. 10) and the chilaria of Limulus (see figs. 13 and 20), may in part represent in the adult the sternum of the excalated praegenital somite. This has not been demonstrated by an actual following out of the development, but the position of these pieces and the fact that they are (in Limulus) supplied by an in-dependent segmental See also:

nerve, favours the view that they may comprise the sternal See also:area of the vanished praegenital somite. This See also:interpretation, however, of the " metasternites " of Limulus and Scorpio is opposed by the coexistence in Thelyphonus (figs. 55, 57 and 58) of a similar metasternite with a complete praegenital somite. H. J. See also:Hansen (10) has recognized that the " praegenital somite " persists in a rudimentary condition, forming a " See also:waist " to the series of somites in the See also:Pedipalpi and Araneae.

The present writer is of See also:

opinion that it will be found most convenient to treat this evanescent somite as something special, and not to See also:attempt to reckon it to either the prosoma or the mesosoma. These will then remain as typically composed each of six appendage-bearing somites—the prosoma comprising in addition the ocular prosthomere.' When the praegenital somite or traces of it are present it should not be called " the seventh prosomatic " or the " first mesosomatic," but simply the " praegenital somite." The first segment of the mesosoma of Scorpio and Limulus thus remains the first segment, and can be identified as such throughout the Eu-arachnida, carrying as it always does the genital apertures. But it is necessary to remember, in the light of See also:recent discoveries, that the See also:sixth prosomatic pair of appendages is carried on the seventh somite of the whole series, there being two prosthomeres or somites in front of the mouth, the first carrying the eyes, the second the chelicerae; also that the first mesosomatic or genital somite is not the seventh or even the eighth of the whole series of somites which have been historically present, See the article ARTHROPODA for the use of the term " prosthomere."but is the ninth, owing to the presence or to the excalation of a praegenital somite. It seems that confusion and trouble will be best avoided by abstaining from the introduction of the non-evident somites, the ocular and the praegenital, into the numerical nomenclature of the component somites of the three great body regions. We shall, therefore, ignoring the ocular somite, speak of the first, second; third, fourth, fifth and sixth leg-bearing somites of the pro-soma, and indicate the appendages by the See also:Roman numerals, I, II, III, IV, V, VI, and whilst ignoring the praegenital somite we shall speak of the first, second, third, &c., somite of the mesosoma or opisthosoma (See also:united mesosoma and metasoma) and indicate them by the Arabic numerals. There are a number of other important points of structure besides those referring to the somites and appendages in which Limulus agrees with Scorpio or other Arachnida and differs from other Arthropods. The See also:chief of these are as follows: I. The See also:Composition of the See also:Head (that is to say, of the anterior part of the pro-soma) with especial Reference to the Region in Front of the Mouth.—It appears (see ARTHROPODA) that there is embryological evidence of the existence of two somites in Arachnids which were originally post-oral, but have become prae-oral by adaptational shifting of the oral aperture. These forwardly-slipped somites are called " prosthomeres." The first of these has, in Arachnids as in other Arthropods, its pair of appendages represented by the eyes. The second has for its pair of appendages the small pair of limbs which in all living Arachnids is either chelate or retrovert (as in spiders), and is known as the chelicerae. It is possible, as maintained by some writers (See also:Patten and others), that the lobes of the cerebral See also:nervous See also:mass in Arach- nids indicate a larger number of prosthomeres as having fused in this region, but there is no embryological evidence at present which justifies us in assuming the existence in Arachnids of more than two prosthomeres. The position of the chelicerae of Limulus and of the ganglionic nerve-masses from which they receive their nerve-See also:supply, is closely similar to that of FiG.

II.—Third leg of Limulus poly-the same structures in phemus, showing the division of the fourth Scorpio. The cerebral segment of the leg by a groove S into mass is in Limulus more two, thus giving seven segments to the easily separat-d by dis- leg as in scorpion. See also:

section as a median See also:lobe (From a See also:drawing by Pocock.) distinct from the laterally- placed ganglia of the cheliceral somite than is the case in Scorpio, but the relations are practically the same in the two forms. Formerly it was supposed that in Limulus both the chelicerae and the next following pair of appendages were prosthomerous, as in See also:Crustacea, but the dissections of Alphonse Milne-Edwards (6) demonstrated soma. M, Mouth behind the oval median camerostome. I, The chelicerae. II, The chelae. VIIIp, The pectiniferous somite. IXstg to Xllstg, the four pulmonary somites. met, The pentagonal metasternite of the prosoma behind all the coxae. x, The sternum of the pectiniferous somite. y, The broad first somite of the See also:meta- the true limitations of the cerebrum, whilst embryological researches have done as much for Scorpio.

Limulus thus agrees with Scorpio and differs from the Crustacea, in which there are three prosthomeres—one ocular and two carrying palpiform appendages. It is true that in the See also:

lower Crustacea (Apus, &c.) we have evidence of the See also:gradual See also:movement forward of the nerve-ganglia belonging to these See also:cox, Coxa or basal segment of the ex', The exopodite of the sixth leg. limb of Limulus. stc, The sterno-coxal process or a, b, c, d, Movable processes on the See also:jaw-like up-growth of the coxa. same leg (see for some sug- epc, The articulated movable gestions on the See also:morphology outgrowth of the coxa, called of this leg, Pocock in Quart. the epi-coxite (present only in Journ. Micr. Sci. See also:March 1901; IV and V of Limulus). explanation). (From Lankester, loc. cit.) palpiform appendages. But although in such lower Crustacea the nerve-ganglia of the third prosthomere have not fused with the anterior nerve-mass, there is no question as to the prae-oral position of two appendage-bearing somites in addition to the ocular prosthomere. The Crustacea have, in fact, three prosthomeres in the head and the Arachnida only two, and Limulus agrees with the Arachnida in this respect and differs from the Crustacea. The central nervous systems of Limulus and of Scorpio present closer agreement in structure than can be found when a Crustacean is compared with either. The wide divarication of the lateral cords in the prosoma and their connexion by transverse commissures, together with the " attraction " of ganglia to the prosomatic ganglion See also:group which properly belong to hinder segments, are very nearly identical in the two animals.

The form and disposition of the ganglion cells are also See also:

peculiar and closely similar in the two. (See Patten (42) for important observations on the neuromeres, &c., of Limulus and Scorpio.) 2. The Minute Structure of the Central Eyes and of the Lateral Eyes.—Limulus agrees with Scorpio not only in having a pair of central eyes and also lateral eyes, but in the microscopic structure of those organs, which differs in the central and lateral eyes respectively. The central eyes are " simple eyes," that is to say, have a single lens, and are hence called " monomeniscous." The lateral eyes are in Limulus " compound eyes," that is to say, consist of many lenses placed close together; beneath each lens is a complex of protoplasmic cells, in which the optic nerve terminates. Each such unit is termed an " ommatidium." The lateral eyes of Scorpio consist of See also:groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus. The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus. It consists of a single layer of cells, continuous with those which secrete the general chitinous covering of the prosoma. The cells of the ommatidium are a See also:good See also:deal larger than the neighbouring See also:common cells of the epidermis. They secrete the knob-like lens (fig. 22). But they also receive the nerve See also:fibres of the optic nerve. They are at the same time both optic nerve-end cells, that is to say, retina cells, and corneagen cells or secretors of the chitinous lens-like cornea.

In Limulus (fig. 23) each ommatidium has a peculiar ganglion See also:

cell developed in a central position, whilst the ommatidium of the lateral eyelets of Scorpio shows small intermediate cells between the larger nerve - end cells. The structure of the lateral eye of Limulus was first described by st Grenacher, and further and more accurately by Lankester and Bourne (5) and by Watase; that of Scorpio by Lan- kester and Bourne, FIG. 13.—Diagrams of the meta-sternite st, who showed that with genital operculum op, and the first lamellithe statements of gerous pair of appendages ga, with uniting von Graber were sternal See also:element st of Scorpio (See also:left) and Limulus erroneous, and (right). that the lateral eyes of Scorpio have a single cell-layered or " monostichous " ommatidium like that of Limulus. Watase has shown, in a very convincing way, how by deepening the See also:pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and See also:Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio. (For details the reader is referred to Watase (11) and to Lankester and Bourne (5).) The structure of the central eyes of Scorpio and spiders and also of Limulus differs essentially from that of the lateral eyes in having two layers of cells (hence called diplostichous) beneath the lens, separated from one another by a membrane (figs. 24 and 25). The upper layer is the corneagen and secretes the lens, the lower is the retinal layer. The mass of soft cell-structures beneath a large lens of a central eye is called an " ommatoeum." It shows in Scorpio and Limulus a tendency to segregate into See also:minor groups or " ommatidia." It is found that in embryological growth the retinal layer of the central eyes forms as a separate pouch, which is pushed in laterally beneath the corneagen layer from the epidermic cell layer. Hence it is in origin double, and consists of a true retinal layer and a post-retinal layer (fig. 24, B), though these are not separated by a membrane.

Accordingly the diplostichous ommatoeum or soft See also:

tissue of the Arachnid's central eye should strictly be called " triplostichous," since the deep layer is itself doubled or folded. The retinal cells of both the lateral and central eyes of Limulus and Scorpio produce cuticular structures on their sides; each such piece is a rhabdomere and a number (five or ten) uniting form a rhabdom (fig. 26). In the specialized ommatidia of the compound eyes of Crustacea and Hexapods the rhabdom is an important structure.' It is a very significant fact that the lateral and central eyes of Limulus and Scorpio not only agree each with each in regard to their monostichous and diplostichous structure, but also in the formation in both classes of eyes of rhabdomeres and rhabdoms in which the component pieces are five or a multiple of five (fig. 26). Whilst each unit of the lateral eye of Limulus has a_rhabdom of See also:tent pieces ' See fig. 12 in the article ARTHROPODA. 2 Though ten is the prevailing number of retinula cells and rhabdomeres in the lateral eye of Limulus, Watase states that they may be as few as nine and as many as eighteen. See also:box st (From Lankester, loc. cit.) 292 forming a See also:star-like chitinous centre in section, each lateral eye of Scorpio has several rhabdoms of five or less rhabdomeres, indicating that the Limulus lateral eye-unit is more specialized than the detached lateral eyelet of Scorpio, so as to present a coincidence of one lens with one rhabdom. Numerous rhabdomeres (grouped as rhabdoms in Limulus) are found in the retinal layer of the central eyes also. Whilst Limulus agrees thus closely with Scorpio in regard to the VII •m VIIopening remains in the adult scorpion. In all the embryonic or permanent opening is on the coxa of the fifth pair of prosomatic limbs.

Thus an organ newly discovered in Scorpio was found to have its counterpart in Limulus. The name " coxal gland " needs to be carefully distinguished from " crural gland," with which it is See also:

apt to be confused. The crural glands, which occur in many terrestrial Arthropods, are epidermal in origin and totally distinct from the coxal glands. The coxal glands of the Arachnida are structures of the same nature as the See also:green glands of the higher Crustacea and the so-called " See also:shell glands " of the See also:Entomostraca. The latter open at the base of the fifth pair of limbs of the Crustacean, just as the coxal glands open on the coxal joint of the fifth pair of limbs of the Arachnid. Both belong to the See also:category of " coelomoducts," namely, tubular or See also:funnel-like portions of the coelom opening to the exterior in pairs in each somite (potentially,) and usually persisting in only a few somites as either "urocoels" (renal organs) or "gonocoels" (genital tubes). In See also:Peripatus they occur in every somite of the body. They have till recently been very generally identified with the nephridia of Chaetopod See also:worms, but there is good See also:reason for considering the true nephridia (typified by the nephridia of the See also:earthworm) as a distinct class of organs (see Lankester in vol. ii. See also:chap. iii. of A See also:Treatise on See also:Zoology, 1900). The genital ducts of Arthropoda are, like the green glands, shell glands and coxal glands, to be regarded as coelomoducts (gonocoels). The coxal glands do not establish any special connexion between Limulus and Scorpio, since thay also occur in the same somite in the lower Crustacea, but it is to be noted that the coxal glands of Limulus are in minute structure and probably in function more like those of Arachnids than those of Crustacea. 4. The Entosternites and their Minute Structure.—Strauss- Durckheim (1) was the first to insist on the affinity between Limulus and the Arachnids, indicated by the presence of a free suspended entosternum or plastron or entosternite in both.

We have figured here (figs. 1 to 6) the entosternites of Limulus, Scorpio and Mygale. Lankester some years ago made a special study of the See also:

histology (3) of these entosternites for the purpose of comparison, and also ascertained the relations of the very numerous muscles which are inserted into them (4). The entosternites are cartilaginous in texture, but they have neither the chemical character nor the microscopic structure of the hyaline See also:cartilage of Vertebrates. They yield chitin in place of chondrin or See also:gelatin—as does also the cartilage of the Cephalopod's endoskeleton. In microscopic structure they all present the closest agreement with one another. We find a See also:firm, homogeneous or sparsely fibrillated See also:matrix in which are embedded pairs of mesosomatic appendages of Scorpio and Limulus compared. first branchial plate of Limulus. stg, Stigma or orifice of the hollow IX, The first pair of lung-books of tendons of the branchial plates of Scorpio and the second branchial Limulus. plate of Limulus. (After Lankester, loe. cit.) eyes, it is to be noted that no Crustacean has structures corresponding to the peculiar diplostichous central eyes, though these occur again (with See also:differences in detail) in Hexapoda. Possibly, however, an investigation of the development of the median eyes of some Crustacea(Apus,See also:Palaemon)may prove them to be diplostichous in origin. 3. The so-called " Coxal Glands."—In 1882 (Prot.

See also:

Roy. See also:Soc. No. 221) Lankester described under the name "coxal glands" a pair of brilliantly See also:white oviform bodies lying in the Scorpion's prosoma immediately above the coxae of the fifth and sixth pairs of legs (fig. 27). These bodies had been erroneously supposed by See also:Newport (12) and other observers to be glandular outgrowths of the alimentary canal. They are really excretory glands, and communicate with the exterior by a very minute aperture on the posterior face of the coxa of the fifth limb on each side. When examined with the See also:microscope, by means of the usual section method, they are seen to consist of a labyrinthine See also:tube lined with peculiar cells, each cell having a deep vertically striated border on the surface farthest from the lumen, as is seen in the cells of some renal organs. The coils and branches of the tube are packed by connective tissue and blood spaces. A similar pair of coxal glands, lobate instead of ovoid in shape, was described by Lankester in Mygale, and it was also shown by him that the structures in Limulus called " See also:brick-red glands " by ----!-- Packard have the same structure and position as the coxal — - - glands of Scorpio and Mygale. In Limulus these organs - consist each of four See also:horizontal lobes lying on the coxal XII _ . margin of the second, third, fourth, and fifth prosomatic limbs, the four lobes being connected to one another by a transverse piece or See also:stem (fig. 28).

Microscopically their structure is the same in essentials as that of the coxal glands of Scorpio (13). Coxal glands have since been recognized and described in other Arachnida. In 1900 it was shown that the coxal gland of Limulus is provided with a very delicate thin-walled coiled duct which opens, even in the adult condition, by a minute See also:

pore on the coxa of the fifth leg (Patten and Hazen, 13A). Previously to this, Lankester's See also:pupil Gulland had shown (1885) that in the embryo the coxal gland is a comparatively simple tube, which opens to the exterior in this position and by its other extremity into a coelomic space. Similar observations were made by Laurie (17) in Lankester's laboratory (189o) with regard to the early condition of the coxal gland of Scorpio, and by Bertkau (41) as to that of the spider Atypus. H. M. See also:Bernard (13B) showed that the all \JUL) (After Lankester, loc. cit.) nucleated cells (corpuscles of See also:protoplasm) arranged in rows of three, six or eight, parallel with the adjacent lines of fibrillation. A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance. On the other hand, the entosternite of the Arachnida is a very large and important feature in the structure of the prosoma, and must See also:play an important part in the See also:economy of these organisms. In Limulus (figs. 1 and 2) it has as many as twenty-five pairs of muscles attached to it, coming - . show the way in which an outgrowing gill - process .

_.. I bearing blood-holding lamellae, may give rise, if the sternal body See also:

wall sinks inwards, to a lung-chamber with air-holding lamellae. I is the embryonic condi- tion. bs, Blood sinus. L is the condition of out-growth with gl, gill lamellae. A is the condition of in-sinking of the sternal surface and consequent enclosure of the lamelligerous surface of the appendage in a chamber with narrow orifice—the pulmonary air - holding chamber. pl, Pulmonary lamellae. bs bs, Blood sinus. (After See also:Kingsley.) to it from the bases of the surrounding limbs and from the dorsal carapace and from the pharynx. It consists of an oblong plate 2 in. in length and 1 in breadth, with a pair of tendinous outgrowths standing out from it at right angles on each side. It " floats " between the prosomatic nerve centres and the alimentary canal. In each somite of the mesosoma is a small, free entosternite having a similar position, but below or ventral to the nerve cords, and having a smaller number of muscles attached to it. The entosternite was probably in origin part of the fibrous connective tissue lying close to the integument of the sternal surface—giving See also:attachment to muscles corresponding more or less to those at present attached to it.

It became isolated and detached, why or with what See also:

advantage to the organism it is difficult to say, and at that See also:period of Arachnidan development the great ventral nerve cords occupied a more lateral position than they do at present. We know that such a lateral position of the nerve cords preceded the median position in both Arthropoda and See also:Chaetopoda. Subsequently to the floating off of the entosternite the approximation of the nerve cords took place in the prosoma, and thus they were able to take up a position below the entosternite. In the mesosoma the approximation had occurred before the entosternites were formed. In the scorpion (figs. 3 and 4) the entosternite has tough membrane-like outgrowths which connect . it with the body-wall, both dorsally and ventrally forming an oblique diaphragm, cutting off the cavity of the prosoma from that of the mesosoma. It was described by Newport as " the diaphragm." Only the central and horizontal parts of this structure correspond precisely to the entosternite of Limulus: the right and left anterior processes(marked ap in figs. 3 and 4, and RAP, LAP, in figs. I and 2) correspond in the two animals, and the median lateral process imp of the scorpion represents the tendinous outgrowths ALR, PLR of Limulus. The scorpion's sf of Limulus See also:polyphemus. The coxae of 293 entosternite gives rise to outgrowths, besides the great posterior flaps, pf, which form the diaphragm, unrepresented in Limulus. These are a ventral arch forming a neural canal through which the great nerve cords pass (figs.

3 and 4, snp), and further a dorsal gastric canal and arterial canal which transmit the alimentary See also:

tract and the dorsal artery respectively (figs. 3 and 4, GC, DR). In Limulus small entosternites are found in each somite of the appendage-bearing mesosoma, and we find in Scorpio, in the only somite of the mesosoma which has a well-developed pair of appendages, that of the pectens, a small entosternite with ten pairs of muscles inserted into it. The supra-pectinal entosternite lies ventral to the nerve cords. In Mygale (figs. 5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio. The anterior notch Ph.N. is similar to that in Limulus, whilst the imbricate triangular pieces of the posterior median region resemble the similarly-placed structures of Limulus in a striking manner. It must be confessed that we are singularly ignorant as to the functional significance of these remarkable organs —the entosternites. Their movement in an upward or downward direction in Limulus and Mygale must exert a pumping See also:action on the blood contained in the dorsal See also:arteries and the ventral See also:veins respectively. In Scorpio the completion of the horizontal plate by oblique flaps, so as to form an actual diaphragm shutting off the cavity of the prosoma from the rest of the body, possibly gives to the organs contained in the anterior chamber a physiological advantage in respect of the supply of arterial blood and its separation from the venous blood of the mesosoma. Possibly the movement of the diaphragm may determine the passage of air into or out of the lung-sacs. See also:Muscular fibres connected with the suctorial pharynx are in Limulus inserted into the entosternite, and the activity of the two organs may be op gapp correlated.

5. The Blood and the Blood- FIG. 19.—Section through an vascular System.—The blood fluids early embryo of Limulus longs-of Limulus and Scorpio are very See also:

spina, showing seven transverse similar. Not only are the blood divisions in the region of the corpuscles of Limulus more like unsegmented anterior carapace. in form and granulation to those The seventh, VII, is anterior to of Scorpio than to those of any the genital operculum, op, and Crustacean, but the fluid is in is the cavity of the praegenital both animals strongly impreg- somite which is more or less nated with the See also:blue-coloured completely suppressed in sub-respiratory proteid, haemocyanin. sequent development, possibly This body occurs also in the blood indicated by the area marked of Crustacea and of Molluscs, but VII in fig. 7 and by the great its abundance in both Limulus entopophyses of the prosomatic and Scorpio is very marked, and carapace. gives to the freshly-See also:shed blood a (After Kishinouye, Jottrn. Sci. See also:Coll. strong See also:indigo-blue tint. See also:Japan, vol. v., rsgz.) The great dorsal contractile See also:vessel or " See also:heart " of Limulus is closely similar to that of Scorpio; its See also:ostia or incurrent orifices are the five pairs of limbs following the chelicerae were arranged in a series on each side between the mouth, M, and the meta' sternites, mets. sgc, Frontal groove. sa, Rudiment of lateral eyes.

obi, Camerostome (upper See also:

lip). so, Sense-organ of Patten. PrGabp', Rudiment of the appendage of the praegenital somite which disappears. abp2, Rudiment of the right See also:half of the genital operculum. ' abpa, Rudiment of the right pecten. abp' to ¢bp', Rudiments of the four appendages which carry the pul- monary lamellae. I to VI, Rudiments of the six limbs of the prosoma. VIIPrG, The evanescent praegenital somite. IX, The second mesosomatic somite or pectiniferous somite. X to XIII, The four pulmoniferous somites. (After Brauer, Zeitsch. viss. Zool. vol. Iix., r8ss.) VIIPrG go abpa - abp' (After Brauer, loc. cit.) See also:cam sf, The sub-frontal median sclerite. M Ch, The chelicerae.

cam, The camerostome or upper lip. M, The mouth. pmst, The promesosternal sclerite of chitinous plate, unpaired. „See also:

mss mets, The right and left metasternites (corresponding to the similarly placed pentagonal sternite of Scorpio). Natural size. (After Lankester.) placed in the same somites as those of Scorpio, but there is one additional posterior pair. The origin of the pai' d arteries from the Ch 294 heart differs in Limulus from the arrangement obtaining in Scorpio, in that a pair of lateral commissural arteries exist in Limulus (as described by Alphonse Milne-Edwards (6)) leading to a suppression of the more See also:primitive See also:direct connexion of the four pairs of posterior N. R. IL c°o On ~„ ,..~Pbe 6c'p n (From Korschelt and Heider, after Laurie.) lateral arteries and of the great median posterior arteries with the heart itself (fig. 29). The arterial system is very completely developed in both Limulus and Scorpio, branching repeatedly until minute arterioles are formed, not to be distinguished from true capillaries; bens these open into irregular swollen vessels which are the veins or venous sinuses. A very remarkable feature in Limulus, first described by Owen, is the close See also:accompaniment of the prosomatic nerve centres and nerves by arteries, so close indeed that the great ganglion mass and its out-See also:running nerves are actually sunk in or invested by (From Korschelt and Heider after Watase.) arteries.

The connexion is not so intimate in Scorpio, but is nevertheless a very close one, closer than we find in any other Arthropods in which the arterial system is well developed, e.g. the See also:

Myriapoda and some of the arthrostracous Crustacea. It seems that there is a primitive tendency in the Arthropoda for the arteries to accompany the nerve cords, and a " supra-See also:spinal " artery—that is to say, an artery in close relation to the ventral nerve cords—has been described in several cases. On the other hand, in many Arthropods, especially those which possess tracheae, the arteries do not have a long course, but soon open into wide blood sinuses. Scorpio certainly comes nearer to Limulus in the high development of its arterial system, and the intimate relation of the anterior aorta and its branches to the nerve centres and great nerves, than does any other Arthropod. An arrangement of great functional importance in regard to the venous system must now be described, which was shown in '883. by Lankester to be common to Limulus and Scorpio. This arrangement has not hitherto been detected in any other class than the Arachnida, and if it should ultimately prove to be peculiar to that group, would have considerable See also:weight as a See also:proof of the close genetic affinity of Limulus and Scorpio. A TV' A, Early condition before the lens is deposited, showing the folding of the epidermic cell-layer into three. B, Diagram showing the nature of this infolding. C, Section through the fully formed eye. h, Epidermic cell-layer. r, The retinal portion of the same which, owing to the infolding, lies between gl, the corneagen or lens-forming portion, and pr, the post-retinal or capsular portion or See also:fold. 1, Cuticular lens.

g, See also:

Line separating lens from the lens-forming or corneagen cells of the epidermis. n, Nerve fibres. rh, Rhabdomeres. [Flow the See also:inversion of the nerve-end-cells and their connexion with the nerve-fibres is to be reconciled with the condition found in the adult, or with that of the monostichous eye, has not hitherto been explained.] (From Korschelt and Heider.) The great pericardial sinus is strongly developed in both animals. Its walls are fibrous and complete, and it holds a considerable See also:volume of blood when the heart itself is contracted. Opening in pairs in each somite, right and left into the pericardial sinus are large veins, which bring the blood respectively from the gill-books and the lung-books to that chamber, whence it passes by the ostia into the heart. The blood is brought to the respiratory organs in both cases by a great venous See also:collecting sinus having a ventral median position. In both animals the wall of the pericardial sinus is connected by See also:vertical muscular bands to the wall of the ventral venous sinus (its lateral ex- pansions around the lung-books in Scorpio) in each somite through which the pericardium passes. There are seven pairs of these venopericardiac vertical muscles in Scorpio, and eight in Limulus (see figs. 30, 31, 32). It is obvious that the contraction of these muscles A B C s. - vim —'.' pv e2~°D*-° u'o 9°-0 0 oey--o W- 4=-42 e pr.

Bl • B through the lateral eye of Euscorpius italicus. lens, Cuticular lens. nerv.e, Retinal cells (nerve-end cells). rhabd, Rhabdomes. nerv. f, Nerve fibres of the optic nerve. int, Intermediate cells (lying between the bases of the retinal cells). (After Lankester and Bourne from See also:

Parker and Haswell's Text-book of Zoology, See also:Macmillan& Co.) must cause a depression of the See also:floor of the pericardium and a rising of the roof of the ventral blood sinus, and a consequent increase of volume and flow of blood to each. Whether the pericardium and the ventral sinus are made to expand simultaneously or all the movement is made by one only of the surfaces concerned, must depend on conditions of tension. In any case it is clear that we have in these muscles an apparatus for causing the blood to flow differentially in increased volume into either the pericardium, through the veins leading from the respiratory organs, or from the body generally into the great sinuses which bring the blood to the respiratory organs. These muscles See also:act so as to See also:pump the blood through the respiratory organs. It is not surprising that with so highly developed an arterial system Limulus and Scorpio should have a highly developed mechanism for determining the flow of blood to the respiratory organs. That this is, so to speak, a need of animals with localized respiratory See also:Ftc.

25.-Section through one of the central eyes of a See also:

young Limulus. L, Cuticular or corneous lens. See also:ret, Retinula cells. hy, Epidermic cell-layer. nf, Nerve fibres. corn, Its corneagen portion See also:im- See also:con. Liss, Connective tissue (mesomediately underlying the lens. blastic skeletal tissue). (After Lankester and Bourne, Q. J. Mie. &i., 1883.) organs is seen by the existence of provisions serving a similar purpose in other animals, e.g. the branchial See also:hearts of the See also:Cephalopoda. The veno-pericardiac muscles of Scorpio were seen and figured by Newport but not described by him. Those of Limulus were described and figured by Alphonse Milne-Edwards, but he called them merely " transparent ligaments," and did not discover their muscular structure. They are figured and their importance for the first time recognized in the memoir on the muscular and skeletal systems of Limulus and Scorpio by Lankester, See also:Beck and Bourne (4).

6. Alimentary Canal and Gastric Glands.—The alimentary canal in Scorpio, as in Limulus, is provided with a powerful suctorial pharynx, in the working of which extrinsic muscles take a part. The mouth is relatively smaller in Scorpio than in Limulus—in fact is minute, as it is in all the terrestrial Arachnida which suck the juices of either animals or See also:

plants. In both, the alimentary canal takes a straight course from the pharynx (which bends under it downwards and backwards towards the mouth in Limulus) to the anus, and is a simple, narrow, cylindrical tube (fig. 33). The only point in which the gut of Limulus resembles that of Scorpio rather than that of any of the Crustacea, is in possessing more than a single pair of ducts or lateral outgrowths connected with ramified gastric glands or gastric caeca. Limi.lus has two pairs of these, Scorpio as many as six pairs. The Crustacea never have more than one pair. The minute microscopic structure of the gastric glands in the two animals is practically identical. The functions of these gastric diverticula have never been carefully investigated. It is very probable that in Scorpio they do not serve merely to secrete a See also:digestive fluid (shown in other Arthropoda to resemble the pancreatic fluid), but that theyalso become distended by the juices of the See also:prey sucked in by the scorpion—as certainly must occur in the case of the simple unbranched gastric caeca of the spiders. The most important difference which exists between the structure of Limulus and that of Scorpio is found in the hinder region of the alimentary canal.

Scorpio is here provided with a single or double pair of renal excretory tubes, which have been identified by earlier authors with the Malpighian tubes of the Hexapod and Myriapod See also:

insects. Limulus is devoid of any such tubes. We shall revert to this subject below. A, Diagram of a retinula of the rhabdom of a retinula of thei scorpion's central eye, showing its five constituent rhabdomeres as rays of a star. D, Transverse section of a retinula of the lateral eye of Limulus, showing ten retinula cells (ret), each bearing a rhabdomere (rhab). (After Lankester.) 7. Ovaries and Spermaries: Gonocoels and Gonoducts The scorpion is remarkable for having the specialized portion of coelom from the walls of which See also:egg-cells or sperm-cells are developed according to See also:sex, in the form of a simple but extensive network. It is not a pair of simple tubes, nor of dendriform tubes, but a closed network. The same fact is true of Limulus, as was shown by Owen (7) I to 6, The bases of the six pro-somatic limbs. A, prosomatic gastric gland (sometimes called salivary). B, Coxal gland. C, Diaphragm of Newport =fib- See also:rous flap of the entosternum.

D, Mesosomatic gastric caeca (so-called See also:

liver). E, Alimentary canal. (From Lankester, Q. J. Mie. Sri., vol. xxiv. N.S. p. 152.) in regard to the ovary, and by Benham (14) in regard to the testis. This is a very definite and remarkable agreement, since such a reticular gonocoel is not found in Crustacea (except in the male Apus). Moreover, there is a significant agreement in the character of the spermatozoa of Limulus and Scorpio. The Crustacea are—with the exception of the Cirrhipedia—remarkable for having stiff, motionless spermatozoids. In Limulus Lankester found (15) the spermatozoa to possess active flagelliform " tails," and to resemble very closely those of Scorpio which, as are those of most terrestrial Arthropoda, are actively motile.

This is a microscopic point of agreement, but is none the less significant. In regard to the important structures concerned with the fertilization of the egg, Limulus and Scorpio differ entirely from one another. L A central ee of a scorpion consisting of five retina-cells (See also:

eel), with adherent branched pigment cells (See also:pig). B, Rhabdom of the same, consisting of five confluent rhabdomeres. C, Transverse section of the The eggs of Limulus are fertilized in the See also:sea after they have been laid. Scorpio, being a terrestrial animal, fertilizes by copulation. The male possesses elaborate copulatory structures of a chitinous nature, and the eggs are fertilized in the See also:female without even quitting the place where they are formed on the wall of the reticular gonocoel. The female scorpion is viviparous, and the young are produced in a highly developed condition as fully formed scorpions. Differences between Limulus and Scorpio.—We have now passed in See also:review the See also:principal structural features in which Limulus agrees with Scorpio and differs from other Arthropoda. There remains for See also:consideration the one important structural difference between the two animals. Limulus agrees with the See also:majority of the Crustacea in being destitute of renal excretory caeca or tubes opening into the hinder part of the gut. Scorpio, on the other hand, in common a' to a5, Posterior See also:borders of the chitinous bases of the coxae of the second, third, fourth and fifth prosomatic limbs.

b, Longitudinal lobe or stolon of the coxal gland. c. Its four transverse lobes or outgrowths corresponding to the four coxae. (From Lankester, loc. cil , after Packard.) with all air-breathing Arthropoda except Peripatus, possesses these tubules, which are often called Malpighian tubes. A great deal has been made of this difference by some writers. It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question), really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes. It must be pointed out that the presence or absence of such renal excretory tubes opening into the See also:

intestine appears to be a question (From Lankester, "Limulus an Arachnid.) of See also:adaptation to the changed physiological conditions of respiration, and not of morphological significance, since a pair of renal excretory tubes of this nature is found in certain Amphipod Crustacea (Talorchestia, &c.) which have abandoned a purely aquatic life. This view has been accepted and supported by Professors Korschelt and Heider (16). An important fact in its favour was discovered by Laurie (17), who investigated the See also:embryology of two See also:species of Scorpio under Lankester's direction. It appears that the Malpighian tubes of Scorpio are developed from the mesenteron, viz. that portion of the gut which is formed by the hypoblast, whereas in Hexapod insects the similar caeca( tubes are developed from the proctodaeum or in :pushed portion of the gut which is formed from epiblast. In fact it is not possible to maintain that the renal excretory tubes of the gut are of one common origin in the Arthropoda. They have appeared independently in connexion with a See also:change in the See also:excretion of nitrogenous See also:waste in Arachnids, Crustacea, and the other classes of Arthropoda when aerial, as opposed to aquatic, respiration has been established—and they have been formed in some cases from the mesenteron, in other cases from the proctodaeum.

Their appearance in the air-breathing Arachnids does not separate those forms from the water-breathing Arachnids which are devoid of them, any more than does their appearance in certain Amphipoda separate those Crustaceans from the other members of the class. Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequently acts in Arthropoda as an organ of nitrogenous excretion in the absence of any special excretory tubules, and that the See also:

production of such caeca from its surface in separate lines of descent does not involve any elaborate or unlikely process of growth. In other words, the Malpighian tubes of the terrestrial Arachnida are homoplastic with those of Hexapoda and Myriapoda, and not homogenetic with them. We are compelled to take a similar view of the agreement between the tracheal air-tubes of Arachnida and other tracheate Arthropods. They are homoplasts (see 18) one of another, and do not owe their existence in the various classes compared to a common See also:inheritance of an ancestral tracheal system. Conclusions arising from the Close Affinity of Limulus and Scorpio.—When we consider the relationships of the various classes of Arthropoda, having accepted and established the fact of the close genetic affinity of Limulus and Scorpio, we are led to important conclusions. In such a consideration we have to make use not only of the fact just mentioned, but of three important generalizations which serve as it were as implements for the proper estimation of the relationships of any series of organic forms. First of all there is the generalization that the relationships of the various forms of animals (or of plants) to one another is that of the ultimate twigs of a much-branching genealogical See also:tree. Secondly, identity of structure in two organisms does not necessarily indicate that the identical structure has been inherited from an ancestor common to the two organisms compared (homogeny), but may be due to See also:independent development of a like structure in two different lines of descent (homoplasy). Thirdly, those members of a group which, whilst exhibiting undoubted structural characters indicative of their proper See also:assignment to that group, yet are simpler than and inferior in elaboration of their organization to other members of the group, are not necessarily representatives of the earlier and primitive phases in the development of the group—but are very often examples of retrogressive change or degeneration. The second and third implements of See also:analysis above cited are of the nature of cautions or checks. Agreements are not necessarily due to common inheritance; simplicity is not necessarily primitive and ancestral.

On the other hand, we must not rashly set down agreements as due to " homoplasy " or " convergence of development " if we find two or three or more concurrent agreements. The probability is against agreement being due to homoplasy when the agreement involves a number of really separate (not correlated) coincidences.

End of Article: RAP

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