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 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.
| After Hubrecht, loc. cit. Fig. 7.—Diagrams of the nervous system of Amphineura. A, Proneomenia. B, Neomenia. C, Chaetoderma. D, Chiton. c, Cerebral ganglia. s, Sublingual ganglia. v, Pedal (ventral) nerve-cord. l, Visceral (lateral) nerve-cord. pc. Post-anal junction of the visceral nerve-cords. | From Gegenbaur, Elements of Comp. Anatomy. Fig. 8.—Anterior part of the nervous system of Chiton cinereus, in more detail. B, Buccal ganglia (concerned with the odontophore). C, Cerebral nerve-mass. P, Pedal ganglion and commencement of pedal nerve-cord. pl, Visceral nerve-cord. The sublingual ganglia are not lettered. |
Fam. 1. Lepidopleuridae.—Terminal margins of end valves never elevated; form oval or oblong. Lepidopleurus cancellatus, Sow. North Atlantic and Mediterranean; various abyssal species. Hanleya hanleyi, Bean, north Atlantic. Hemiarthrum Microplax. The extinct Gryptochitonidae, Pilsbry, with other Palaeozoic genera, narrow and elongated in form with terminal margins of end valves elevated, belong to this group.
Suborder II. Mesoplacophora, Pilsbry.—Insertion plates well developed and slit.
Fam. 2. Ischnochitonidae.—All the valves with slits, and the inner layer well covered by the outer.
Subfam. 1. Ischnochitoninae.—No shell-eyes: sutural laminae separated; slits in the valves 1-7 do not correspond with the ribs of the tegmentum. Ischnochiton, Trachydermon, Chaetopleura, Stenoplax, Stenoradsia.
Subfam. 2. Callochitoninae. With shell-eyes and united sutural laminae. Callochiton laevis, North Atlantic and Mediterranean.