Encyclopaedia Britannica, 11th Edition, "Chicago, University of" to "Chiton" / Volume 6, Slice 2

by Various

A. Lepidopleurus benthus. B. Boreochiton cinereus.

C. Schizochiton incisus. a, anus; f, foot; g, gills; m, mouth; pa, mantle; pa’, anal lobe of mantle; ps, pallial slit; te, pallial tentacles.

Fig. 4.—Diagrams of the alimentary canal of Amphineura (from Hubrecht). A. Neomenia and Proneomenia. B. Chaetoderma. C. Chiton. o, Mouth. a, Anus. d, Alimentary canal. l, Liver (digestive gland).

The tegmentum is formed by the fold of mantle covering the 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, containing 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 is often a numerical asymmetry 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 digestive gland, consisting of two lobes which are symmetrical in the young animals, but in the adult the right lobe is anterior and smaller.

Fig. 5.—Diagrams of the excretory and reproductive organs of Amphineura (after Hubrecht).

A, Chaetoderma. B, Neomenia. C, Proneomenia. D, Chiton. O, Ovary. P, Pericardium. N, Nephridium.

u, External aperture of nephridium. g, External aperture of the genital duct of Chiton. r, Rectum. Cl, Cloacal or pallial chamber of Neomeniae and Chaetoderma. Br, Ctenidia (branchial plumes).

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 laterally and ventrally, and open into the tube (fig. 6). The sexes are distinct, and the ovary is frequently greenish in colour, the testis red. The gonad is transversely wrinkled and lies between the aorta and the intestine, extending from the pericardium to the anterior end of the body. A simple gonaduct on each side arises 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 the genital 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.

After Haller (Arbeiten zool. Instit.), Vienna, 1882. Fig. 6.—Dissection of the renal organs (nephridia) of Chiton siculus. F, Foot. 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. eg, Stem of the nephridium leading to no, its external aperture. nk, Reflected portion of the nephridial stem. ng, Fine caeca of the nephridium, which are seen ramifying transversely over the whole inner surface of the pedal muscular mass.