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CYCLOSTOMATA, or MARSIPOBRANCHII
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See also:CYCLOSTOMATA, or MARSIPOBRANCHII , a See also:group of fishes including the See also:ordinary lampreys and hagfish, and so called from the wide permanently gaping mouth which is without the hinged jaws characteristic of other vertebrates (GNATHOSTOMATA). 0 The class Cyclostomata consists of two orders, the Myxinoids (or Hyperotreti) and the Petromyzontes (or Hyperoartii), which, while showing sufficient resemblance in structure to See also:warrant their inclusion in the same class, are yet marked off by such deep-seated See also:differences as to indicate that they commenced to diverge from one another far back in evolutionary See also:time. The See also:order Myxinoids includes the hagfish (Myxine), See also:common off the eastern, and occurring also, though less commonly, off the western coasts of the See also:north See also:Atlantic, and the genus Bdellostoma (also known as Homea, Eptatretus, in part—Polistotrema), including the " borers " of the western See also:American See also:coast, New See also:Zealand and the Cape of See also:Good See also:Hope. The order Petromyzontes includes the widely distributed lampreys. The See also:original genus Petromyzon (which it is now customary to subdivide into a number of genera) includes the large See also:sea See also:lamprey (P. See also:marinus) of the north Atlantic coasts and the two fresh-See also:water lampreys of See also:European streams (P. fluviatilis and P. planeri, the latter of which is possibly only a small-sized variety of the former See also:species). In North See also:America nine or ten species of lampreys are known to occur, descriptions of which are given by See also:Jordan and Evermann (1). In the See also:southern hemisphere occur the two genera Mordacia (See also:Chile, See also:Tasmania) and Geotria (Chile, See also:Australia, New Zealand) (2). The Cyclostomes are remarkable among vertebrates in that they are semiparasitic in See also:habit. The lampreys—except some of the small fresh-water forms—attach themselves to other fishes by their suctorial mouth and proceed to rasp off the flesh by means of the horny See also:teeth carried by the highly-See also:developed See also:tongue. The Myxinoids have gone a step further and actually See also:bore their way right into the See also:body of their See also:prey, devouring all the soft parts and leaving the skin behind as a See also:mere See also:shell, empty but for the bones. Where the hagfish or borers are abundant, as in certain localities off the See also:east coast of See also:Scotland and off the See also:west coast of See also:California, they may do See also:great damage to See also:fisheries from their habit of attacking fishes which are in difficulties through being caught by a See also:hook or in a See also:net; the See also:fish when See also:drawn up being frequently completely deprived of their flesh. The Myxinoids retain the ancestral marine See also:habitat, but the lampreys have sought See also:refuge from the struggle for existence by taking to fresh water to a less or greater extent. Such a See also:form as Petromyzon marinus or Entosphenus tridentatus of the west coast of America is what is known as anadromous in habit, i.e. it takes refuge in fresh water during the breeding See also:season, ascending See also:rivers like the See also:salmon for the purpose of spawning. Certain species of lampreys, on the other See also:hand, have completely deserted the sea and spend their whole lives in fresh-water streams or lakes. The See also:lake lampreys show a See also:reminiscence of their ancestral migratory habits in leaving lakes and ascending streams in order to See also:deposit their spawn. See also:Anatomy.—In structural features, the Cyclostomes show a curious mixture of features which must be looked on as See also:primitive with others which are indicative of high specialization for their See also:peculiar mode of See also:life. In See also:general See also:appearance they are" See also:eel-like ": they are elongated in shape and adapted for See also:swimming in eel See also:fashion, i.e. the body is propelled forward by the backward passage along it of waves of lateral flexure. There are, however, certain conspicuous differences which at once serve to distinguish a Cyclostome from any other fishes of eel-like shape:—(r) the circular permanently open mouth, (2) the See also:absence of all trace of paired limbs, (3) the absence of paired See also:external nasal openings, and (4) the presence on the roof or at the tip of the See also:head of a conspicuous median opening—the pituitary opening. It will be convenient, in describing the structural features of the group, to take as a basis for the description the marine lamprey, Petromyzon marinas. A marine lamprey is an eel-like creature 70 to 75 cm. in length. At the anterior end and situated somewhat ventrally is the circular widely gaping mouth or buccal cavity, its lining studded with sharply pointed See also:thorn-like " teeth " and its edge fringed with numerous sensory papillae. On the dorsal See also:side of the head is the conspicuous circular pituitary opening with prominent lips, while on the sides are seen the eyes, and behind these a See also:row of somewhat rounded branchial openings or gill-clefts. At about the beginning of the posterior See also:fourth of the body, and in the midventral See also:line, is the anal opening, and immediately behind it is the prominent papilla carrying the opening of the urogenital sinus. The hinderportion of the body, in accordance with its See also:function in locomotion, is flattened from side to side, while its See also:surface is increased by the development of a median fin See also:fold, divided, except in See also:early stages of development, into three portions, known as the first and second dorsal fins and the caudal fin. The last mentioned is of the primitive protocercal type. The whole surface of the body—which shows a conspicuous dark marbling, especially dorsally, on a See also:light ground—is covered with highly glandular epidermis. An important feature is the See also:complete absence of all trace of the calcified placoid plates which are so characteristic of the Elasmobranchii. The Myxinoids differ from the lampreys in regard to several of the above-mentioned characters. The edges of the mouth carry tentacle-like barbels. The pituitary opening is See also:close to the anterior edge of the mouth opening instead of being right up on the dorsal side of the head. The eyes are invisible, being greatly reduced and sunk far below the surface, and in Myxine, though not in Bdellostoma, the row of gill openings is represented by a single opening on each side nearly in the midventral line and situated at about the end of the first See also:quarter of the body length. Ventrally the Myxinoid possesses on each side of the body a row of remarkable epidermal glands which can produce at will enormous quantities of glutinous slime. This secretion, which, no doubt, is of much value as a See also:protection from attack, is composed of very See also:fine threads, formed by the See also:conversion of the See also:protoplasm of certain cells of the epidermal glands (" See also:thread cells ") into an extremely fine, tightly coiled filament, which becomes unwound when discharged to the exterior. Pituitary See also:Tube.—A remarkable peculiarity of the Cyclostomes lies in the fact that the pituitary ingrowth of ectoderm does not, as in other forms, become involved in the inpushing of ectoderm From D. Starr Jordan, .4 See also:Guide to the Study of Fishes, by permission of A. See also:Constable & Co., Ltd. i.—The Marine Lamprey (Petromyzon marinas, L.). which forms the buccal cavity. On the contrary, it lies outside the edge of the stomodaeum, and in the See also:case of the lampreys active growth takes See also:place in the See also:tissue between the pituitary and stomodaeal ingrowths, so that the two openings come to be widely separated, the pituitary opening being pushed back on to the dorsal side of the head. The pituitary opening remains patent throughout life, as is the case with Crossopterygians alone amongst Gnathostomata. In Myxine a further remarkable peculiarity in regard to the hypophysis, probably adaptive in nature, occurs, inasmuch as the pituitary invagination develops an opening at its posterior end into the pharynx. See also:Nervous See also:System.—The anterior end of the nervous tube is enlarged and differentiated to form a See also:brain as in other Vertebrates, but this brain in the lampreys at least shows remarkably primitive features. The enlargement as compared with the See also:spinal See also:cord is seen to be comparatively slight: the brain is much elongated, and its various regions See also:lie in a straight line one behind the other: the roof of the brain retains to a great extent the primitive See also:epithelial See also:condition. On each side anteriorly there is See also:present a comparatively large olfactory See also:lobe, and this is continued posteriorly into a small cerebral hemisphere. The lampreys are amongst those vertebrates in which there is an See also:eye-like apparatus (3) connected with the roof of the thalamencephalon. There grow out from the roof of the thalamencephalon two processes, a posterior (the pineal See also:process), and an anterior (the parapineal process). The pineal process grows forwards so as to overlie the parapineal process. Each of these projections from the roof of the thalamencephalon dilates to form a vesicle, and each vesicle shows certain eye-like characteristics, its deep See also:wall forming a " retina " and its superficial wall being clear and translucent (" pellucida "). The retinal cells are packed in the case of the pineal See also:organ with opaque See also: Alimentary See also:Canal.—The widely gaping buccal See also:funnel is morphologically an inpushing of the See also:outer skin, i.e. it is stomodaeal in nature. The thorn-like teeth which See also:stud its lining are formed simply by cornification of the epidermal cells (4) like the provisional horny teeth of a See also:tadpole, and are not homologous with the true teeth of ordinary vertebrates. As to whether they represent the remnant of a once present system of epidermal scales, which may have preceded the coating of placoid elements in the See also:evolution of the vertebrate, there is no See also:evidence. The pharyngeal region, closely associated with the See also:respiratory function, possesses, on each side, a See also:series of gill-sacs (six in Myxine: seven in Petromyzon, besides an anterior one which is laid down in the embryo but disappears later: up to as many as fourteen in Bdellostoma) opening on the one hand to the pharynx and on the other to the exterior. In Bdellostoma and in the larva of Petromyzon lf. br. See also:pit. N best Pr. i.j.v. a.v.e. h.v. Modified from T. J. See also:Parker, Zootomy, fig. 4, by permission of See also:Macmillan & Co., Ltd. a.v.o, Atrio-ventricular opening. oes, See also:Oesophagus. br, Brain. See also:olf, Olfactory organ. br.o, See also:Internal opening of gill See also:sac. pc, Pericardium. d.a, Dorsal aorta. p.c.v, See also:Left posterior See also:cardinal vein. d.c, Ductus cuvieri. pit, Pituitary tube. h.v, Hepatic vein. V, Ventricle. i.j.v, Inferior jugular vein. v, Velum. N, Notochord. the gill'-sacs open directly from the pharynx to the exterior, but in the adult lamprey and in Myxine the original relations are modified. In Myxine, the external openings of the gill-sacs have migrated backwards along the side of the body and become coincident at a point slightly posterior to the last sac. It follows from this that each sac is connected with the common See also:aperture by a tube, longest in the case of the first sac, shortest in the case of the last. In the adult lamprey a different modification is found. Here the dorsal portion of the pharynx has become nipped off as a narrow tube which functions as an oesophagus from the larger ventral portion, which forms an elongated saccular structure ending blindly at its hinder end and having in its lateral wall the internal openings of the gill-sacs. Breathing.—The inspiratory current passes inwards by the mouth opening in the larval lamprey, by the pituitary tube in Myxine, while in the adult lamprey both expiration and See also:inspiration takes place through the external gill-openings. In the case of the lampreys the elastic See also:skeleton of the branchial region (see below) plays an important See also:part in respiration. The branchial region shows rhythmic contraction through the agency of the transverse muscles—and expansion, through the See also:elasticity of the branchial skeleton—in the adult lamprey. These rhythmic movements of the branchial region cause successive inflow and outflow through the branchial openings. In the larva, on the other hand, the respiratory current always passes in one direction—backwards. This is helped by the presence of a velar fold at the front end of the pharynx, which acts as a See also:valve opening only backwards, and to the presence of membranous flaps projecting back from the anterior border of each gill-opening and acting as valves which open only outwards. Behind the pharynx comes the truly See also:digestive part of the alimentary canal in the form of a straight tube showing little differentiation into special regions. The lining 'of the See also:intestine is increased in See also:area by an inwardly projecting fold, which is compared by some morphologists with the See also:spiral valve of certain other See also:groups. In themature See also:river lamprey the digestive See also:tract becomes in great part degenerate. Coelomic Organs.—The chief point of See also:interest about the splanchnocoele or perivisceral cavity is that in the Myxinoids the adult shows a persistent embryonic condition in that the pericardiac portion never becomes isoiated from the maain body cavity. The renal organs are of special interest in the Myxinoids from their very See also:simple See also:character. The See also:kidney duct is seen See also:running along the roof of the coelom on either side. Into the duct open See also:short segmentally arranged tubes, each possessing at its closed rounded extremity a Malpighian body. Each of these short tubes is morphologically a nephric tubule, which, however, in correlation with its shortness, is without the turns and twists so characteristic of such tubules generally. A further consequence of the short simple character of the tubules is that they are quite See also:separate from one another, instead of being massed together to form a compact gland such as the kidney is elsewhere. In Petromyzon the kidney has the ordinary compact form, and here also the Malpighian bodies are shut off from the splanchnocoele. The ovary or testis is a large unpaired structure See also:hanging from the dorsal wall of the splanchnocoele and shedding its products into it; from the coelomic space the genital products pass into the urogenital sinus—formed by the See also:fusion of the kidney ducts at their hinder ends —through a small opening, one at each side. This opening, which leads directly from coelom into urogenital sinus, is known as the genital See also:pore. Its morphological significance is doubtful. Skeleton.—The vertebral See also:column of the lamprey is represented by a persistent notochord surrounded by a thick sheath, which shows no signs of invasion by See also:cartilage cells or of segmentation. Resting on the sheath are paired dorsal See also:arch elements, more numerous than the neuromuscular segments. In the tail region these are See also:united into a continuous See also:band of cartilage on each side: similar cartilaginous bands represent the ventral arch elements of the tail region. The skeleton of the head region consists of a cartilaginous cranium, into the formation of which enter typical parachordal and trabecular elements, together with olfactory and auditory capsules. In addition to these, there are a number of other cartilaginous pieces present in the head region, the homologies of which are doubtful. Branchial See also:Basket.—One of the most characteristic features of the skeleton of the lamprey is the remarkable cartilaginous " branchial basket," which supports the gill region. In an adult river lamprey the basketwork consists on each side of a series of eight See also:vertical See also:half-hoops of cartilage. The hoops of each side are connected together dorsally by a pair of See also:longitudinal bars, lying ventral to the notochord, and ventrally by a similar pair of rods which are fused in the See also:middle line. Slender cartilaginous projections arise from the anterior and posterior sides of the hoops, and certain of these See also:meeting at their ends form additional longitudinal bars connecting together successive hoops. Connected with the basketwork posteriorly is a remarkable See also:cup-shaped cartilage, which supports the hind wall of the pericardium. The series of cartilaginous half-hoops naturally suggest the half-hoops of cartilage which form the skeleton of the visceral See also:arches in the Gnathostomata. They are, however, more superficial in position, and this has led many to doubt their actual homology with the cartilaginous visceral arches. Taking into See also:account, how-ever, our present knowledge of the development of the two sets of structures, it seems on the whole probable that a true homology exists and that the branchial basket of the lamprey represents merely a set of visceral arches modified in accordance with the peculiar breathing methods of the creature. In the Myxinoids the branchial basket is reduced to a few vestigial masses of cartilage. Vascular System.—The See also:heart (5) of the lamprey consists of an See also:atrium and a single ventricle, the atrium on the left, the ventricle on the right. Into the atrium, on its right side, and behind the atrio-ventricular opening, there opens a nearly vertical chamber usually termed the sinus venosus (see below), the opening guarded by a pair of vertically placed valves. The ventricle passes anteriorly into what is clearly the homologue of the conus arteriosus of other forms. In its interior are present a pair of laterally placed longitudinal ridges similar to the ridges which occur in other forms in the conus. The opening from ventricle into conus is guarded by a pair of laterally placed See also:pocket valves situated just within the boundary of the ventricle. The arterial system is of the ordinary piscine type. From the heart there passes forwards a ventral aorta, split into two separate vessels in its anterior half, and giving off on each side a series of efferent vessels to the gill-sacs, one passing between each two gill-sacs and an additional one to the front wall of the front sac and to the posterior wall of the last. The See also:blood is collected from the walls of the gill-sacs by a series of efferent vessels which open into the dorsal aorta. It is to be noted that the dorsal aorta retains the probably primitive unpaired condition, except for a very short extent at its anterior end, where it is split so as to form two short aortic roots. Venous System.—The See also:main venous channels are like those in other fishes. though their connexion with the heart becomes modified in the adult. The two posterior cardinals—with their continuations forwards, the anterior cardinals—approach the median See also:plane and undergo fusion in the region of their opening into the t*o ductus Cuvieri. The left ductus Cuvieri then atrophies so that all the blood from the cardinals reaches the heart by way of the originally right ,uiii~i~See also:Ill~IiIIIIIIIIIINIIIIIIiIIIInVIIIIIiIIhIIVIIIIIIIIICIBIIillll(IIIIIIIII!""'0°!~Idlllllllllllllllllllllllllillllll ductus Cuvieri. It is this right ductus Cuvieri which forms the dorsal part of what is usually termed the sinus venosus. The inferior jugular See also:veins which return the blood from the ventral side of the head also become replaced in the adult by a median unpaired vein which opens posteriorly into the sinus venosus by what probably represents the hinder end of the original right inferior jugular. It is interesting to note that in Polyfterus, one of the Crossopterygian Fanoids, there is a somewhat similar asymmetrical condition of inferior jugulars and ductus Cuvieri. Oviposition of Lamprey (6).—The lamprey chooses as spawning ground a part of the stream with fairly rapid current and where the bottom is composed of See also:sand with scattered stones. By means of the suctorial mouth, stones are removed from more or less circular area so as to form a shallow excavation. The male and See also:female frequently See also:work together at the task of preparing the See also:nest. When oviposition is about to take place, the male may be seen to suddenly attach himself to the dorsal surface of the head of the female which holds on to one of the stones at the upper margin of the nest. The urogenital opening of the male, with its specially prominent papilla, is approximated to that of the female, and with a peculiar quivering See also:movement the eggs and sperms are emitted synchronously amidst clouds of sand stirred up by the movements of the tail. The eggs fertilized thus at the moment of exit are very sticky from their coating of albumen, and become weighted down by adherent grains of sand. Development.—The development of the lamprey is of much morphological importance from the archaic nature of the creature and from the fact that the See also:egg is comparatively small (about 1 mm. in See also:diameter), so that development is not greatly modified by a large See also:mass of yolk. It has been worked out so far only in the river lamprey (7). Segmentation is complete and unequal. It, as well as the process of gastrulation, agrees in its main features with the same phenomenon in Amia, Dipnoans and Urodele amphibians. The blastopore persists as the anal opening of the adult. The mesoderm arises in a manner closely comparable with that which occurs in A mphioxus, the chief difference being that the mesoderm segments are solid instead of hollow, except in the anterior head region, where they are true hollow enterococlic pouches. The rudiment of the central nervous system has the form of a solid See also:keel-like ingrowth of ectoderm along the See also:mid-dorsal line, which only secondarily becomes hollowed out—just as happens in Teleostean fishes. The See also:young lamprey, after completing its embryonic development, passes three or four years, in fact its whole life up to the time of sexual maturity, in a prolonged larval condition in which its structure shows important differences from that of the adult. This larval See also:stage of the fresh-water lamprey of See also:Europe was See also:long supposed to be a separate genus of Cyclostomes and was called Ammocoetes. The Ammocoetes lives in the mud and breathes and feeds by means of a current of water produced by ciliary See also:action, which carries Flagellates and other microscopic organisms in through the mouth opening. Correlated with this mode of feeding the buccal cavity is without the teeth so characteristic of the adult. A number of complicated branched sensory processes grow into and nearly occlude the cavity, forming a See also:kind of See also:sieve with only narrow chinks through which the ingoing current passes. The water passes out by the gill openings, which in Ammocoetes open See also:direct from pharynx to exterior. Certain arrangements of the pharyngeal wall of Ammocoetes show a remark-able resemblance to what is found in See also:Amphioxus. The See also:thyroid, which in the adult is a complicated ductless gland, has in the young Ammocoetes the form of a longitudinal groove of the ventral wall of the pharynx. This groove is lined by columnat cells, some carrying See also:cilia, others being glandular and secreting sticky slime. These gland cells are arranged in four longitudinal bands. The thyroid is, in fact, in this stage in a condition corresponding exactly with the endostyle of Amphioxus. The agreement extends to function the secretion, forming sticky threads which entangle See also:food particles. Anteriorly a pair of peripharyngeal bands pass dorsalwards, one on each side, to See also:bend back suprapharyngeal bands which are continued to the hinder end of the pharynx. Here again the resemblance to what occurs in Amphioxus is very close. The Ammocoetes possesses a functional See also:liver with bileduct, while in the adult river lamprey the alimentary canal is degenerate. It has no arch elements on its notochord. lts eyes are sunk beneath the surface and nonfunctional, and they retain to a great extent an embryonic character (8). There is a rapid process of See also:metamorphosis from the larval to the adult condition, the details of which are by no means sufficiently known. After the metamorphosis the now mature lamprey accomplishes the See also:act of See also:reproduction and then uparently See also:dies almost immediately. The development of the yxinoids is much less well known than that of the lampreys. As regards the common hagfish (Myxine glutinosa), we are indeed still in complete See also:ignorance in regard to its developmental See also:history in spite of persistent efforts to obtain embryological material. It seems probable that during the breeding See also:period the hagfishes retire into some particularly inaccessible habitat. Within the last few years, however, abundant material illustrating the developmental history of Bdellostoma (9) has been obtained off the Californian coast, and this when fully worked out will give us a-good See also:idea of the general lines of Myxinoid development. The egg differs greatly from that of the lampreys. It is—as is that of Myxine—of large size, richly689 yolked and of a shortened-up sausage shape. It See also:measures about 22 mm. by 8 mm. Surrounding the egg is a protective See also:capsule of a yellow horny appearance. At one end a cap-like portion of this forms a detachable operculum, in the middle of which is a See also:minute opening, the micropyle. Each end of the capsule is prolonged into a group of stiff processes with See also:anchor-like expansions at their tips. Segmentation is, as in other richly yolked eggs, incomplete, confined to the germinal disk at the opercular See also:pole. The central nervous system in Bdellostoma develops by the overarching of medullary folds, not out of a solid keel as is the case with the lampreys. History in Time.—The softness of the skeletal tissues and the absence of scales in Cyclostomata provide little opportunity for the preservation of fossil remains of this group, and no known fossils can be referred with certainty to the Cyclostomata. The Devonian See also:Palaeospondylus gunni has been regarded as a Cyclostome by some authors, but this relationship is at the least doubtful. Other authors have associated the See also:Ostracoderms, the See also:oldest known vertebrates, with this group. Additional information and CommentsThere are no comments yet for this article.
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