Glossophora, with a foot, which, except in certain swimming forms, is simple, median, and flattened into a broad, sole-like surface by the contractions of which the animal crawls. It is often divided into three successive regions, pro-, meso-, and meta-podium, by lateral constrictions.
1 Wagner (op. cit.) states that Clio possesses, (1) a short copulatory organ containing a male receptaculum seminis; (2) a long excitatory organ; both of them in- and e-vaginable. When two individuals meet, they perforate reciprocally each other's body, with the excitatory organ, from which masses of granules are discharged into the coelome. They separate, after having mutually filled the male receptacula. Congress of each of these two individuals with an ovigerous individual is necessary for the fertilisation of the ova, the sperm received on the first occasion being now transferred.
Gastropoda, in which the primitive bilateral symmetry is retained not only in the head and foot, fact also in the mantle and visceral dome. The anus is posterior. The antero-posterior axis, joining month and anus, is long: the dorso-ventral (dorso-pedal) axis short. The pedal and visceral nerve-cords are straight, parallel with one another, and extend the length of the body: ganglionic enlargements are developed feebly or not at all. The circulatory organs, ctenidia, nephridia, and genital ducts are paired and bi-laterally symmetrical.
The epidermis consists of a single layer of cells, with a cuticula in which microscopic calcareous spines (Chaetoderma), spicules, or plates (Neomeniae) are imbedded in connection apparently with the cells of the epidermis. In Polyplacophora (Chiton, etc.) similar calcareous spines and plates, and in some instances chitinous spines, occur on the margins of the mantle, whilst a series of eight calcareous shell-plates covers the median dorsal region. These plates articulate one with the other, and their outer margins are overlapped by a fold of the integument. The foot is narrow and long in Polyplacophora, very narrow and inclosed in a groove in Neomeniae, and only indicated in the posterior part of the body in Chaetoderma. It is ciliated in the two last-named groups, and it has a foot-gland opening below the mouth in Neomeniae. The mantle fold is well developed in Polyplacophora, and surrounds both head and foot. In the Neomeniae and Chaetoderma it is reduced to a collar surrounding the anus, unless the edges of the groove inclosing the foot may be regarded as its homologue as well.
The head is not prominent: it is partially surrounded by a projecting fold in Polyplacophora, the homologue of the tentacles and cephalic lobes or lips of Anisopleura. The central nervous system consists in the Polyplacophora of a nervous ring situated at the base of the cephalic fold, and connected to two pairs of lateral cords, one pair, the visceral cords, extending down the sides of the body and united posteriorly above the anus; the other pair, the pedal cords, extending down the median line in the foot. The whole of this system is orange in colour, and consists of both nerve-fibres and ganglion-cells. There is a pair of buccal ganglia and a pair of sub-radular ganglia connected with the cephalic ring. The pedal cords are connected across the median line by numerous and irregular transverse fibrous commissures. The Neomeniae have distinct cerebral enlargements, and in Proneomenia the pedal as well as the visceral cords are connected posteriorly. Transverse pedal commissures are present; and in Proneomenia transverse commissures between the pedal and visceral cords 1 Chaetoderma has a bi-lobed cerebral enlargement, from which the pedal and visceral cords take origin.
These cords unite together posteriorly, and the common cord thus formed on each side unites with its fellow above the anus. There are no pedal commissures, and the single ring surrounding the oesophagus appears to correspond with the buccal ring. The nerve-cords contain ganglion-cells, as in Polyplacophora. Sensory organs lodged in cavities of the shell-plates have been described in the last-named order, and in some genera eyes as well. The latter may be exceedingly numerous, and are furnished with a transparent cornea, pigmented envelope, lens and retina.
1 One or two similar connections have been found in Chiton.
The mouth is more or less ventral in position. There is a muscular pharynx. A radula is absent in Neomenia, present and furnished with very complex transverse rows of teeth in Polyplacophora, with many teeth in Proneomenia, and a single tooth in Chaetoderma. A remarkable sensory sub-radular organ is found in Chiton. Small lobed buccal (salivary ?) glands are present in Chiton, and a pair of long salivary tubes in Proneonienia. Two saccular glands open into the oesophagus in Chiton (? all species). Their epithelium undergoes remarkable colour changes during secretion, and they are amylolytic, or salivary, in function. The intestine is straight in Neomeniae and Chaetoderma; ciliated in the former, and provided with lateral liver caeca, with a single liver caecum in Chaetoderma. It is convoluted and ciliated in Polyplacophora, and there is a right and left liver caecum, both lobulated and furnished with acini. The rectum opens on a papilla in Polyplacophora, between the two branchial plumes in Chaetoderma, and in union with the nephridial opening in Neomeniae. The heart is situated posteriorly in a pericardial cavity.
It consists of a ventricle and right and left auricle in Polyplacophora: and, so far as is known, only of a simple ventricle in other Isopleura. There is a dorsal aortic trunk running forward. There are apparently no specialised branchiae in Proneomenia. They lie near the anus in the mantle cavity, and are tufted in Neomenia, paired in Chaetoderma. In the Polyplacophora they have the form of small ctenidia, arranged in the mantle furrow at the side of the foot, and numbering sixteen or more on each side. The nephridia open into the pericardial cavity. They have the form of short wide sacs with a common external aperture in Neomeniae into the anus, or separate openings, one on each side the anus, in Chaetoderma, and in these two orders they serve as genital ducts. They are long, extend forwards, and are folded upon themselves in Chiton, the folded portion bearing numerous secretory acini; and they open externally on each side in front of the genital apertures and in the mantle furrow. The sexes are separate in Polyplacophora and Chaetoderma, whereas the Neomeniae appear to be hermaphrodite. The single ovary and testis are alike in Polyplacophora, and are simple glands with a right and left duct opening posteriorly into the mantle furrow.
The hermaphrodite gland of Proneomenia appears to be double. In it, in Neomenia and Chaetoderma, the genital duct or ducts open into the pericardial cavity, and through it communicate with the nephridia. Neomenia carinata has been said to possess lateral male ducts with calcareous penes.
In Chiton the ova are impregnated in the pallial groove. Segmentation is total, and at first equal. There is a Veliger with a flagellum in the centre of the velar area, a pair of lateral eyes in the mantle furrow, and a foot-gland which aborts. The nerve-cords are formed from the ectoderm. Transverse dorsal furrows make their appearance, in which the cuticula thickens, and the thickenings are the rudiments of the shell-plates. There does not appear to be a shell-gland.
The sub-class Isopleura is divisible into three orders: - (I) Polyp lacophora (Chiton and its allies); (2) Neonieniae, including the genera Neomenia and Proneo-menia; and (3) Chaetoderma, with a single genus of the same name.
Polyfilacophora. Chitons of Adriatic, Haller, Arb. Zool. Inst. Wien, iv. 1883; v. 1884. Haddon, Challenger Reports, xv. 1886. Eyes and sensory organs, Moseley, Q. J. M. xxv. 1885. Nephridium, etc, Sedgwick, P. R. S. xxxiii. 1883; Van Bemmelen, Z. A. vi. 1883; cf. Haller, M. J. xi. p. 4I. Eggs and envelopes, Sabatier, Revue Sc. Nat. Paris and Montpellier (3), iv. Development, Kowalesky, Annales Mus. Nat. Hist. Marseilles, i. 1883.
For other literature and that relative to Neonieniae and Chaetoderma, see Hubrecht on Amphineura, Q. J. M. xxii. 1882. This author is stated to be preparing a monograph of the group in the 'Fauna and Flora of the Gulf of Naples.'