Conus, Nassa, Bithynia. Other special glands are the grapelike poison glands of Aplysia and Dolabella, which open near the genital aperture close to the osphradium, and the small glands on the inner surface of the mantle-edge which secrete a purple liquid in Aplysia Camelus, a colourless liquid in other species of the genus and its allies. In Phyllirhoe certain unicellular cutaneous glands exude a fatty phosphorescent secretion. The muscles are composed of long non-striated muscle-cells. The foot is pre-eminently muscular, and certain of its muscles - the columellar muscles - connect the animal to its shell. The head, tentacles, buccal mass, and even in some instances the nervous collar surrounding the latter, have special retractor muscles, connected to or derived from the columellar muscles. Calcareous spicules are found in the dorsal integument of Doris, etc. among Non-Palliata. Thread cells (nematocysts) are found in sacs at the apices of the cerata or dorsal processes, in some Non-Palliata, e. g.
The cerebral ganglia are sometimes closely approximated, sometimes far apart. The pleural and pedal ganglia usually lie below and at the sides of the buccal mass, but are fused together surrounding the aorta cephalica in stylommatophorous Pulmonata: and in the Non-Palliata the right and left ganglia appear to be fused with the corresponding cerebral ganglia, but remain connected by a slender loop beneath the oesophagus. In Fissurella and Haliotis (Zygobranchia), in Turbo (Trochidae) the pedal ganglia are replaced by two long cords extending down the foot, sheathed with ganglion cells and connected by transverse commissures. The pleural ganglia are only incompletely separated from these cords anteriorly. The ganglia or the cords replacing them are often of a deep orange colour. The cerebro-pedal and cerebro-pleural connectives are sometimes of great length, and distinct from one another; sometimes short, and contained within a common sheath. The two main pedal nerves are connected by transverse commissures in Paludina among Azygobranchia and some Pulmonata (p. 120). The two visceral nerves connected with the pleural ganglia are always united posteriorly, but the length of the loop thus formed and the distinctness of its two visceral and the single abdominal ganglia are variable.
In one section of the Anisopleura, the Streptoneura, the posterior union of the visceral nerves, which are always long, is situated dorsally to the intestine, and the loop is therefore twisted with the torsion of the visceral dome, the right side of the loop passing above, the left below the intestine. In the other section, the Enthyneura, the union is situated ventrally to the intestine, and the nerves are consequently not twisted, and in this case the loop may be very short, yet the ganglia remain distinct, e. g. in Limnaeus among basommatophorous Pulmonata. A pair of buccal ganglia is invariably present, connected with the cerebral ganglia: but in Fissurella, Haliotis, and Turbo their connectives have been traced to the pleuro-pedal centres. Ganglia may also be present on other nerves.
Sensory cells with sense-hairs occur on the tentacles of the head, and of other parts of the body, and the body itself. A number of these cells sometimes form projecting 'gustatory' papillae. In Fissurella and certain species of Trochus lateral organs, composed of a central mass of sense cells with surrounding supporting cells, are found ventral to the tentacles of the mantle furrow. Special ganglia are situate at the bases of these lateral tentacles, and ganglion cells beneath the lateral organs. A sensory organ known as osphradium lies near the ctenidium. It is present in the Limpet (Patella), and in some Pulmonata, e.g. Lininaeus, where the ctenidium is aborted. It consists of a patch of cylindrical ciliated cells, with an underlying ganglion, and is always supplied with a nerve from the visceral loop or ganglion. There are two osphradia in the Zygobranchia, one right, the other left. Gustatory buds have been detected in the oral cavity of Fissurella and Trochus. The eyes are usually two in number, situated one at the base of each cephalic tentacle, and sometimes raised on a papilla. In certain Pulmonata, hence Stylommatophora, they are placed at the tips of the two superior tentacles.
In some Non-Palliata, e.g. Eolis, Doris, and the palliate Opfsthobranch Philine, they lie upon the cerebral ganglia, and are then small in size. In Patella the eye is a cup-shaped depression, widely open; in Haliotis, some species of Trochus, nearly closed; in other Anisopleura quite closed. The retina is formed of a single layer of cells, differentiated into pigmented and non-pigmented. The latter are broadest at their bases, narrow at their inner ends, which are prolonged into a delicate rod. The former are broadest at their inner ends, which are furnished with rods, narrow at their bases, and they surround, in groups containing four to eight cells, each non-pigmented cell, the visual rod of which is enveloped by their rods. Both (?) kinds of cells are connected by basal processes with the fibres of the optic nerve, which spread over the back of the eye, and in many instances at least contain intercalated ganglion cells. In Helix there is a well-developed peripheral optic ganglion (Carriere). The open retinal capsule contains a vitreous body in Haliotis, (? Patella); the closed a vitreous body and a lens, which occupies the anterior part of the capsule. The former is soft, the latter more dense and slightly yellow; both are structureless.
A vitreous body is said to be absent in stylom-matophorous Pulmonata (Hilger and others)1. The internally-placed eye of Opisthobranchia appears to be much simplified. The anterior epithelial cells of the closed retinal capsule are transparent, as is also the thin layer of connective tissue intervening between it and the superficial epithelium of the surface or. cornea. In the Natantia the whole eye is inclosed within a capsule, to which it is attached by muscles. Certain species of Onchidium (Pulmonata), littoral marine slugs, possess a number of retractile tentacles ranged along the dorsum. Each tentacle bears two to three eyes. These eyes possess a lens composed of five cells, a retina in which the visual rods are turned externally, i.e. away from the lens, and an optic nerve which perforates the retina, the nerve-fibres being distributed on its inner surface. The two otolithic vesicles are usually in close apposition with the pedal ganglia, but in Natantia and Non-Palliata near the cerebral ganglia, from which their nerve is invariably derived.