CRUSTACEA , a very large See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

None has the See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

Perhaps the most important of all Crustacea, however, with respect to the part which they See also:

• PLAY

See also:

• LATREILLE
• LATREILLE, PIERRE ANDRE (1762–1833)

The same See also:

• LINE

A dorsal andaventral See also:

• PLATE

The " growing point " of the See also:

• TRUNK (Fr. tronc, Lat. truncus, cut off, maimed)

somite corresponding to the eyes (the morphological nature of which is discussed below), the smallest number of head-somites so See also:

• UNITED

Throughout the whole of the Malacostraca the thorax consists of eight and the abdomen of six somites (fig. 4), and the two regions are sharply distinguished by the character of their appendages. In the various groups of the Entomostraca, on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

It may assume the form of a bivalve shell entirely enclosing the body and limbs, as in many ep 14 See also:

• EA(nar...), (p.s„,.,.)
• EA(psr...), (p, „...)• (pqr...)

6) and in the Ostracoda. In the Cirripedia it forms a fleshy See also:

• MANTLE

It seems likely that a similar explanation is to be applied to the coalescence of one or two trunk-somites with the head in the Copepoda, and, if this be so, the only Crustacea remaining in which no trace of a shell-fold is found in the adult are the Anostracous Phyllopoda such as Branchipus (fig. 5, 5). See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

In a Phyllopod such as Apus the limbs of the trunk consist of a flattened, unsegmented or obscurely segmented See also:

• AXIS (Lat. for " axle ")

This modification is usually only found in the antennules and antennae, but it may exceptionally be found in the appendages of the trunk, as, for instance, in the thoracic legs of some Decapods (e.g. Mastigocheirus). Very often one or other of the appendages may be modified for prehension, the seizing of prey or the holding of a See also:

• MATE (a corruption of make, from O. Eng. gemaca, a " comrade ")
• MATE, or PARAGUAY TEA

When Highly magnified view of one of the protruded from the opening cirri. (Morse.) of the shell these " cirri " are spread out to form a casting-See also:

• NET

When the eye-stalk is removed from a living lobster or See also:

• PRAWN

The functions of the antennae are more varied than is the case with the antennules. In many Entomostraca (Phyllopoda, Cladocera, Ostracoda, Copepoda) they are important, and sometimes the only, organs of locomotion. In some male Phyllopoda they form complex " claspers " for holding the female. They are frequently organs of attachment in parasitic Copepoda, and they may be completely pediform in the Ostracoda. In the Malacostraca they are chiefly sensory, the endopodite forming a long flagellum, while the exopodite may form a lamellar " See also:

• SCALE
• SCALE (1) A

The maxillulae and maxillae (or, as they are often termed, first and second maxillae) are nearly always flattened leaf-like appendages, having gnathobasic lobes or endites borne by the segments of the protopodite. The endopodite, when present, is unsegmented or composed of few segments and forms the " palp," and outwardly-directed lobes representing the exopodite and epipodites may also be present. These limbs undergo great modification in the different Froups. The maxillulae are sometimes closely connected with the paragnatha '' or lobes of the lower lip, when these are present, and it has been suggested that the paragnatha are really the basal endites which have become partly separated from the rest of the appendage. The limbs of the post-cephalic series show little differentiationamong themselves in many Entomostraca. In the Phyllopoda they are for the most part all alike, though one or two of the anterior pairs may be specialized as sensory (A pus) or grasping (Estheriidae) organs. In the Cirripedia (Thyrostraca) the six pairs of biramous cirriform limbs differ only slightly from each other, and in many Copepoda this is also the case. In other Entomostraca considerable differentiation may take place, but the series is never divided into definite "tagmata " or groups of similarly modified appendages. It is highly characteristic of the Malacostraca, however, that the trunk-limbs are divided into two sharply defined tagmata corresponding to the thoracic and abdominal regions respectively, the limit between the two being marked by the position of the male genital openings. The thoracic limbs have the endopodites converted, as a rule, into more or less efficient walking-legs, and the exopodites are often lost, while the abdominal limbs more generally preserve the biramous form and are, in the more primitive types, natatory. These tagmata may again be subdivided into groups preserving a more or less marked individuality. For example, in the Amphipoda (fig.

4) the abdominal appendages are constantly divided into an anterior group of three natatory " swimmerets " and a posterior group of three limbs used chiefly in See also:

• JUMPING

In the Isopoda the respiratory function has been taken over by the abdominal append-ages, both rami or only the inner becoming thin or flattened. In the Decapoda the branchial See also:

• SYSTEM

In a few Entomostraca (some Phyllopoda and Ostracoda) the chitinous lining of the fore-gut develops spines and hairs which help to triturate and See also:

• STRAIN (through O. Fr. straindre, estraindre, mod. i treindre, from Lat. stringere, to draw tight, related to stress, stretch, string, &c.)

The its muscular coat ; g, g, the See also:

• HEART
• HEART, LUNG

Excretory System.—The most important excretory or renal organs of the Crustacea are two pairs of glands lying at the base of the antennae and of the second maxillae respectively. The two are probably never functional together in the same animal, though one may replace the other in the course of development. Thus, in the Phyllopoda, the antennal gland develops See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

See also:

• NERVOUS

In the Malacostraca it is absent in the adult, or persists only in a vestigial condition, as in some Decapoda and Schizopoda. It is typically tripartite, consisting of three See also:

• CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

This is the case in the Cladocera, the Cumacea and a few Amphipoda. Mention should also be made of the partial or complete See also:

• ATROPHY (Gr. It- priv., rpo4, nourishment)

In the Mysidae among the Schizopoda a pair of similar otocysts are found in the endopodites of the last pair of appendages (uropods). These contain each a single concretionary otolith. Recent observations, however, make it very doubtful whether aquatic Crustacea can hear at all, in the proper sense of the term, and it has been shown that one function, at least, of the so-called otocysts is connected with the equilibration of the body. They are more properly termed statocysts. Another modification of sensory setae is supposed to be associated with the sense of See also:

• SMELL (connected etymologically with " smoulder " and " smoke ")

In some See also:

• FRESHWATER

Where the sexes are distinct, a more or less marked dimorphism often exists. The male is very often provided with clasping organs for seizing the female. These may be formed by the modification of almost any of the appendages, often the antennules or antennae or some of the thoracic limbs, or even the mandibular palps (some Ostracoda). In addition, some of the appendages in the neighbourhood of the genital apertures may be modified for the purpose of transferring the genital products to the female, as, for instance, the first and second abdominal limbs in the Decapoda. In the higher Decapoda the male is generally larger than the female and has stronger chelae. On the other hand, in other groups the male is often smaller than the female. In the parasitic Copepoda and Isopoda the disparity in size is carried to an extreme degree, and the minute male is attached, like a See also:

• PARASITE (From Gr. irapa, beside, Tiros food)

The gonads, as in other Arthropoda, are hollow saccular organs, the cavity communicating with the efferent ducts. They are primitively paired, but often coalesce with each other more or less completely. The ducts are present only as a single pair, except in one genus of parasitic Isopoda (Ilemioniscus), where two pairs of oviducts are found. Various accessory structures may be connected with the efferent ducts in both sexes. The oviducts may have diverticula serving as receptacles for the spermatozoa (in cases where internal impregnation takes place), and may be provided with glands secreting envelopes or shells around the eggs. The male ducts often have glandular walls, secreting capsules or spermatophores within which the spermatozoa are packed for transference to the female. The terminal part of the male ducts may be protrusible and act as an intromittent organ, or this function may be discharged by some of the appendages, as, for instance, in the Brachyura. The position of the genital apertures varies very greatly in the different groups of the class. They are farthest forward in the case of the female organs of the Cirripedia, where the openings are on the first thoracic (See also:

• FOURTH

In some Phyllopoda (A pus) See also:

• EGG (O.E. aeg, cf. Ger. Ei, Swed. aegg, and prob. Gr. u,ov, Lat. ovum)
• EGG, AUGUSTUS LEOPOLD (1816-1863)

They are of interest, however, rather from the point of view of general embryology than from that of the special student of the Crustacea, and cannot be fully dealt with here. Segmentation is usually of the superficial or centrolecithal type. The hypoblast is formed either by a definite invagination or by the See also:

• IMMIGRATION (from Lat. in, into, and tnigrare, to depart)

12) has an See also:

• OVAL (Lat. ovum, egg)

At successive moults the somites increase in number, new,somites being added behind those already differentiated, from a formative See also:

• ZONE (Gr. 'WV77, a girdle, from 'wvvLvay to gird)

The dots indicate the actual size. within the egg (or within the maternal brood-chamber), so that the larva, on hatching, has reached a stage more advanced than the nauplius. Further, the See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

It is most typically developed in the most specialized Decapoda, the Brachyura, while the more primitive groups of Malacostraca, the Euphausiidae, Penaeidea and Stomatopoda, retain the primitive order of appearance ca its second stage. (See also:

• SPENCE, THOMAS (1750-1814)

Past History. Although fossil remains of Crustacea are abundant, from the most See also:

• ANCIENT
• ANCIENT (also spelt ANTIENT; derived, through the Fr. ancien, old, from the late Lat. antianum, from ante, before)

The Caridea or true prawns and shrimps appear later, in the Upper Jurassic, some of them presenting primitive characteristics in the retention of swimmingexopodites on the walking-legs. The Eryonidea (fig. 16, 3), a group related to the Loricata but of a more generalized type, are specially interesting since the few existing deep-sea forms appear to be only surviving remnants of what was, in the Mesozoic period, a dominant group: The Mesozoic Glyphaeidae have been supposed to stand in the direct line of descent of the See also:

• MODERN

; See also:

• LONDON

As regards the nauplius, however, the constancy of its general character in the most widely diverse groups of Crustacea strongly suggests that it is a very ancient type, and the view has been advocated that the Crustacea must have arisen from an unsegmented nauplius-like ancestor. ' The objections to this view, however, are considerable. The resemblances between the Crustacea and the Annelid worms, in such characters as the structure of the nervous system and the mode of growth of the somites, can hardly be ignored. Several structures which must be attributed to the common stock of the Crustacea, such as the paired eyes and the shell-fold, are not present in the nauplius. The opinion now most generally held is that the primitive Crustacean type is most nearly approached by certain Phyllopods such as Apus. The large number and the uniformity of the trunk somites and their appendages, and the structure of the nervous system and of the heart in A pus, are Annelidan characters which can hardly be without significance. It is probable also, as already mentioned, that the leaf-like appendages of the Phyllopoda are of a primitive type, and attempts have been made to refer their structure to that of the Annelid parapodium. In many respects, however, the Phyllopoda, and especially A pus, have diverged considerably from the primitive Crustacean type. All the cephalic appendages are much reduced, the mandibles have no palps, and the maxillulae are vestigial. In these respects some of the Copepoda have retained characters which we must regard as much more primitive. In those Copepods in which the palps of the mandibles as well as the antennae are biramous and natatory, the first three pairs of appendages retain throughout life, with little modification, the shape and function which they have in the nauplius stage, and must, in all likelihood, be regarded as approximating to those of the primitive Crustacea. In other respects, however, such as the See also:

• ABSENCE (Lat. absentia)

In order to reconstruct the hypothetical ancestral Crustacean, therefore, it is necessary to combine the characters of several of the existing groups. It may be supposed to have approximated, in general form, to Apus, with an elongated body composed of numerous similar somites and terminating in a caudal furca; with the post-oral appendages all similar and all bearing gnathobasic processes; and with a carapace originating as a shell-fold from the maxillary somite. The eyes were probably stalked, the antennae and mandibles biramous and natatory, and both armed with masticatory processes. It is likely that the trunk-limbs were also biramous, with additional endites and exites. Whether any of the obscure fossils generally referred to the Phyllopoda or Phyllocarida may have approximated to this hypothetical form it is impossible to say. It is to be noted, however, that the Trilobita, which, according to the classification here adopted, are dealt with under See also:

• ARACHNIDA

The Copepoda must have separated themselves very early, though perhaps some of their characters may be persistently larval rather than phylogenetically primitive. The Cirripedia are so specialized both as larvae and as adults that it is hard to say in what direction their origin is to be sought. For the Malacostraca, it is generally admitted that the Leptostraca (Nebalia, &c.) provide a connecting-link with the base of the Phyllopod See also:

• STEM (O. Eng. staefn, stemn, cf. Du. stain, Ger. Stamm, &c., probably related to " staff ")

It may be useful to give here a synopsis of the classification adopted in this See also:

• ENCYCLOPAEDIA

Sub-orders Brachyura. Macrura. Orders Schizopoda (including Anaspides). Stomatopoda. Sympoda (Cumacea). Isopoda (including Tanaidacea). Amphipoda. (W. T.