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Encyclopedia Britannica - Main :: CHA-CHR |
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CHITON , the name 1 given to fairly common littoral animals of rather small size which belong to the phylum Mollusca, and, in the possession of a radula in the buccal cavity, resemble more especially the Gastropoda. Their most important characteristic in comparison with the latter is that they are, both in external and internal structure, bilaterally symmetrical. The dorsal integument or mantle bears, not a simple shell, but eight calcareous plates in longitudinal series articulating with each other. The ventral surface forms a flat creeping " foot," and between mantle and foot is a pallial groove in which there is on each side a series of gills. Originally the Chitons were placed with the limpets, Patella, in Cuvier's Cyclobranchia, an order of the Gastropoda. In 1876 H. von Jhering demonstrated the affinities1 The Gr. xtrwv was a garment in the shape of a loose tunic, varying at different periods: see CosTUME: Greek. of Neomenia and Chaetoderma, vermiform animals destitute of shell, with the Chitons, and placed them all in a division of worms which he named Amphineura. The discovery by A. A. W. Hubrecht in 1881 of a typical molluscan radula and odontophore in a new genus Proneomenia, allied to Neomenia, showed that the whole group belonged to the Mollusca. E. Ray Lankester (Ency. Brit., 9th ed., 1883) placed them under the name Isopleura as a subclass of Gastropoda. Paul Pelseneer (19(36) raised the group to the rank of a class of Mollusca, under von Jhering's name Amphineura. The Amphineura are divided into two orders: (1) the Polyplacophora, or Chitons; (2) the Aplacophora, or forms without shells, Neomenia, Chaetoderma and their allies. Order I.POLYPLACOPHORA Each of the eight valves of the shell is made up of two distinct calcareous layers: (a) an outer or upper called the tegmentum, which is visible externally; (b) a deeper layer called articula- :1rr m di` .od C A. Dorsal view of Chiton "Was- these, and all round the nessenksii, Midd., showing animal, is the mantle-skirt. the eight shells. (After (After Cuvier.) Middendorf.) C. The same species of Chiton, B. View from the pedal surface with the shells removed and of a species of Chiton from the dorsal integument re- the Indian Ocean. p, foot; fleeted. b, buccal mass; in, o, mouth (at the other end retractor muscles of the of the foot is seen the anus buccal mass; ov, ovary; raised on a papilla); kr, od, oviduct; i, coils of in- oral fringe; br, the numer- testines; ao, aorta; c', left ous ctenidia (branchial auricle; c, ventricle. plumes) ; spreading beyond mentum which is porcellaneous, quite compact, and entirely covered by the tegmentum. In the lower forms the two layers are coextensive and have smooth edges, but in the higher forms (Moseley). the articulamentum projects laterally beyond and beneath the tegmentum into the substance of the mantle. These projections are termed insertion plates; they are usually slit or notched to form teeth, the edges of which may be smooth and sharp
The tegmentum is formed by the fold of mantle covering the A B C From Lankester, Treatise on Zoology .position of gills. A. Lepidopleurus benthus. mouth; pa, mantle; pa', B. Boreochiton cinereus. anal lobe of mantle; ps, C. Schizochiton incisus. a, pallial slit; te, pallial anus; f, foot; g, gills; m, tentacles. edge of the articulamentum, and extends over the latter from the sides. It is the first part of the shell formed in development. The tegmentum is much reduced in Acanthochiton, and absent in the adult Cryptochiton. The tegmentum is pierced by numerous vertical ramified canals which contain epithelial papillae of the epidermis. These papillae form pallial sense-organs, tcontaining nerve-end bulbs, covered by a dome of cuticle, and innervated from the pallial nerve-cords. They are termed according to their size, micraesthetes and megalaesthetes. In the common species of Chiton and many others of the family Chitonidae the megalaesthetes are developed into definite eyes, the most complicated of which have retina, pigment within the eye, cornea and crystalline lens (intra-pigmental eyes) (fig. 2). The eyes are arranged in rows running diagonally from the median anterior beak of each valve to its lateral borders There may be only one such row on either side, or many rows. In some species the total number present amounts to thousands.Branchiae.The series of gills may extend the whole length of the body in the pallial groove, or may be confined to the posterior end. Each gill has the structure of a typical molluscan ctenidium, consisting of an axis bearing an anterior and posterior row of filaments or lamellae. The gills are thus metamerically repeated; there may be from four to eighty pairs, but there ip 0 A. Neomenia and Proneomenia. B. Chaetoderma. C. Chiton. o, Mouth. a, Anus. d, Alimentary canal. 1, Liver (digestive gland).A, B, C, D, 0, P, N, u, often a numerical 2symmetry on the two sides. The largest pair of branchiae is placed immediately behind the renal openings and corresponds to the single pair of other molluscs, the organs being repeated anteriorly only (Metamacrobranchs) or anteriorly and posteriorly (Mesomacrobranchs). Intestine.The digestive tube in the Polyplacophora, which are herbivorous, is longer than the body, and thrown into a few coils, the anus being median and posterior. The mouth leads into the buccal cavity, on the ventral side of which opens the radular caecum. Each transverse row of teeth of the radula contains 17 teeth, one of which is median, while the second and the fifth on each side are enlarged. Two pairs of glands open into the buccal cavity, and at the junction of pharynx and oesophagus is another pair called the sugar glands. The stomach is surrounded by the liver or digestiveBr -g Chaetoderma. Neornenia. Proneomenia. Chiton. Ovary. Pericardium. Nephridium. External aperture ridium. gland, consisting of two lobes which are symmetrical in the young animals, but in the adult the right lobe is anterior and smaller. Coelom, Gonads and Excretory Organs.As in other molluscs the coelom is represented by a large pericardial cavity, situated above the intestine posteriorly, and a generative sac which is single and median and situated in front of the pericardium, except in the Nuttalochiton hyadesi, where the gonads are in a similar position, but are paired. The excretory organs are coelomoducts with an internal ciliated opening into the pericardium and an opening to the exterior. Both the openings are close together, the external opening being just in front of the principal gill near the posterior end of the body. The renal tube is doubled on itself, its middle part where the bend occurs being situated more or less anteriorly. The excretory surface is increased by numerous ramified caeca which extend beneath the body wall
249 from the gonad near its posterior end and passes first forwards, then backwards, and lastly outwards to the external opening in the pallial groove, anterior to the renal aperture. There may be from one to nine gills between thegenital and renal pores. Heart and Vascular System.The heart is enclosed in the pericardium, and consists of a median elongated ventricle and a pair of lateral auricles, so that the structure somewhat resembles that in the Lamellibranchiata. The openings of the auricles into the ventricle vary in different forms. In many of the lower forms (Lepidopleuridae, Mopalidae, Ischnochitonidae) the opening on each side is single and anterior. In the true Chitonidae there are generally two apertures on each side, and in two species three or four, another instance of the tendency to metameric repetition in the group. The auricles are connected with one another posteriorly behind the ventricle. The ventricle leads into a single anterior median aorta. As in other molluscs, the arteries do not extend far, but lead into inter-visceral blood-spaces. The venous blood is conducted from the tissues to a large sinus on either side above the pallial groove, and from this sinus passes to the gills by an afferent vessel in each gill on the internal or pedal margin of the axis. The oxygenated blood is carried from each gill by an efferent vessel on the external or pallial side of the axis to another longitudinal vessel which leads to the auricle on each side. Nervous System.There are no well-marked specialized ganglia in the central nervous system, nerve-cells being distributed uniformly along the cords. There are two pairs of longitudinal cords, a pedal pair situated ventrally and united beneath the intestine by numerous commissures, and a pallial pair situated laterally and continuous with one another above the rectum (fig. 7). The four cords are all connected anteriorly with the cerebral commissure which lies above the buccal mass anteriorly. From the points where the cords meet the cerebral commissure, arise on each an anterior labial commissure and a stomatogastric commissure. The letter bears two ganglion swellings, the buccal ganglia. The labial commissure gives off a subradular commissure which also bears two ganglia, these being in close relation to a special
jection with nerve-endings, nklying in front of the radula and probably gustatory in function. One osphradium or branchial olfactory organ is usually present on each side, on either side of the anus on the inner wall
Development. The eggs may be laid separately in-vested by a chitinous envelope, or as in Ischnochiton magdatenensis they may form strings containing nearly 200,000 eggs, or the ova may be retained in the pallial groove and undergo development there, as in Chiton polii and Hemiarthrum setulosum. One species Callistochiton viviparus is viviparous and its ova develop without a larval stage in the maternal oviduct. Segmentation is total and at first regular, and is followed by invagination, the blastopore passing to the position of the future mouth. By the development of a ciliated ring lust in front of the mouth the embryo becomes a trochosphere. In the centre of the praeoral Iobe is a tuft of cilia. Just behind the ciliated ring is a pair of larval eyes which disappear in the adult; these correspond to the cephalic eyes of Lamellibranchs. An ectodemic invagination forms a large mucous gland on the foot, which is more or less atrophied in adult life. The gonads originate by proliferation of the anterior wall of the pericardium. .The shell. A S C External aperture of the genital duct of Chiton. Rectum. Cloacal or pallial chamber of Neomeniae and Chaetoderma. Ctenidia (branchial plumes). g, r, Cl, Br, of neph- After Halle
L, Edge of the mantle not removed in the front part of the specimen. s.o., Oesophagus. af, Anus. gg, Genital duct. go, External opening of the same. Stem of the nephridium leading to no, its external aperture. Reflected portion of the nephridial stem. Fine caeca of the nephridium, which are seen ramifying trans. versely over the whole inner surface of the pedal muscular mass. eg, nk, tag, valves arise as transverse thickenings of the dorsal cuticle behind the ciliated ring, the tegmentum being the first part formed. Classification. Suborder I. EOPLACOPHORA, Pilsbry.Tegmentum coextensive with articulamentum, or the latter projecting in smooth unslit plates. Fam. 1. Lepidopleuridae.Terminal margins of end valves never elevated; form oval or oblong. Lepidopleurus cancellatus, Sow. North Atlantic and Mediterranean; various abyssal species. Ilanleya hanleyi, Bean, north Atlantic. Hemiarthrum Microplax. The extinct Gryptochi-B tonidae, Pilsbry, with other Palaeozoic genera, narrow and elongated in form with terminal margins of end valves elevated, belong to this group. Suborder II. MESOPLACO- End of Article: CHITON If you wish, you can link directly to this article.
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