Nervous System.—Ganglionic enlargements are more conspicuous than in the Chitons. In front of the buccal mass is a median cerebral ganglion. From this pass off two pairs of cords, the pleural and pedal, in Proneomenia separate from their origin, in Neomenia united at first and diverging at a pleural ganglion. The pedal cords anteriorly form a pair of pedal ganglia united by a thick commissure. The supra-rectal commissure may be present and bear an ovoid ganglion; or may be wanting. With regard to sense organs the epithelial papillae of the mantle have been mentioned. There is also in some genera a median retractile sensory papilla on the dorsal posterior surface above the rectum, not covered by the cuticle.
Development has only been described in Myzomenia banyulensis, by G. Pruvot. It closely resembles in the early stages that of Chitons. The external surface of the trochosphere is formed of a number of ciliated test-cells. The ectoderm behind the ciliated ring develops spicules, and the post-oral region of the larva elongates. Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface. This appears to indicate that the Neomeniomorpha are descended from Chiton-like ancestors, and that they have lost their shell valves.
Classification of the Neomeniomorpha.—Fam. 1. Lepidomeniidae. Slender, tapering behind, with subventral cloacal orifice; thin cuticle without papillae; flattened spicules; no gills. Lepidomenia, Ismenia, Ichthyodes, Stylomenia, Dondersia, Nematomenia, Myzomenia, M. banyulensis, Mediterranean and Plymouth.
Fam. 2. Neomeniidae. Short, truncate in front and behind; cloacal orifice transverse; gills present; rather thin cuticle; no radula. Neomenia (N. carinata, N. Atlantic and N. and N.W. Scotland), Hemimenia.
Fam. 3. Proneomeniidae. Elongated, cylindrical, rounded at both ends; thick cuticle with acicular spicules; radula polystichous or wanting. Proneomenia, Amphimenia, Echinomenia, Rhopalomenia (R. aglaopheniae, Mediterranean and Plymouth), Notomenia, Pruvotia, Strophomenia.
Fam. 4. Parameniidae. Short and truncated in front; thick cuticle, often without papillae; gills and radula present. Paramenia, Macellomenia, Pararhopalia, Dinomenia, Cyclomenia, Proparamenia, Uncimenia, Kruppomenia.
| Fig. 10.—Chaetoderma nitidulum, Lovén (after Graff). The cephalic enlargement is to the left, the anal chamber (reduced pallial chamber, containing the concealed pair of ctenidia) to the right. |
Suborder II. Chaetodermomorpha.—Aplacophora without distinct ventral groove, with single median unisexual gonad, with differentiated hepatic sac, and with cloacal chamber furnished with two bipectinate gills. There are only two genera in this suborder: Chaetoderma, and Limifossor from Alaska. The characters therefore are very uniform. The body is worm-like and cylindrical, the posterior half a little thicker than the anterior; the posterior extremity forms the enlarged funnel-like branchial or cloacal chamber. The anterior extremity is also somewhat enlarged. The whole surface is uniformly covered with short compressed calcareous spicula embedded in the cuticle.
Branchiae.—The single pair of branchiae are placed symmetrically right and left of the anus, and each has the structure of a ctenidium bearing a row of lamellae on each side as in the Polyplacophora.
Intestine.—The mouth is anterior, terminal and crescentic, and beneath it is a rounded ventral shield. On the floor of the pharynx or buccal mass is a rudimentary radula, which in many species consists of a single large tooth, bearing two small teeth or a row of teeth. In other species the radula is more of the usual type consisting of several transverse rows of two or three teeth each. Two pairs of salivary glands open into the buccal cavity. The digestive tube is straight and simple, wider in its anterior part, into which opens the duct of the hepatic caecum (fig. 4, B). The latter extends backwards on the ventral side of the intestine.