Page:EB1911 - Volume 06.djvu/265

 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.

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.

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 sense-organ called the subradular organ, an epithelial projection with nerve-endings, lying 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 of the mantle, near the base of the last gill. In Lepidopleuridae an osphradium occurs at the base of each gill. The sense organs of the shell-valves have already been described.

Development.—The eggs may be laid separately invested by a chitinous envelope, or as in Ischnochiton magdalenensis 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 just in front of the mouth the embryo becomes a trochosphere. In the centre of the praeoral lobe 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-valves