Dissected so as to show its nervous system.
THE collar has been divided to the right of the pulmonary aperture, to the left of its columellar lobule, which is left in situ on the right side. The attachment of the mantle below the collar to the integument of the body-has been divided from right to left, and again along the right margin of the pulmonary chamber. The mantle-fold or roof of the pulmonary chamber thus freed from its connections has been turned over to the left and displays the terminal portion of the intestine at its edge, the pulmonary vessels, the triangular kidney, and the pericardium, the latter opened to show the heart. The integument covering the head and neck has been divided in the middle line, and the floor of the pulmonary chamber continuous with it removed. The whole of the viscera have also been removed, leaving only the buccal mass, the nerve-collar and the columellar muscles cut short. The buccal mass is much retracted, as it is when the animal's head is drawn in. At its base are seen, the origins of the oesophagus and salivary ducts above, and the sac of the radula below. A stout black bristle has been passed under the right cerebral ganglion.
From the anterior edge of this ganglion a nerve passes to the right upper tentacle which bears the eye: a similar nerve, not visible here, passes to the lower tentacle. From its posterior edge a nerve passes along the buccal mass to a small stellate ganglion, the buccal ganglion, which lies below the salivary duct and innervates the buccal mass; a fine black bristle has been placed beneath it. There are two such ganglia, right and left, connected by a commissure below the oesophagus. The right cerebral is united to the left by a commissure passing across the buccal mass: it is united to the infra-oesophageal ganglion below the buccal mass by a broad band. This band contains two nerve-connectives (not visible here), one to the anterior or pedal portion, the other to the posterior or visceral (parieto-splanchnic) portion of the infra-oesophageal ganglion. From the posterior margin of this ganglion three nerves pass backwards to the body walls; a fourth, which accompanies the aorta cephalica, has been removed together with that vessel. Bundles of nerves may be seen passing down from the anterior or pedal portion of. the ganglion to the foot. The cut ends of the columellar muscles are visible in a bundle below the buccal mass.
These muscles are attached on the one hand to the columella of the shell, on the other to the foot, to the buccal mass, tentacles, and nerve-collar on either side. They therefore serve as retractors of these organs.
There are two pairs of cephalic tentacles in nearly all terrestrial Pulmonatay the superior pair bearing the eyes at their apex: hence the name Stylommatophora applied to this section of the order. They are hollow, and in- and e-vaginable. The exceptions are the Australian genera Janella and Aneitea, in which the eye-bearing tentacles alone are present, and the slugs Onchidium and Vaginulus, in which the tentacles are solid but extremely contractile. The aquatic Pulmonata, on the other hand, have but one pair of tentacles, solid and contractile, with the eyes placed at the inner side of their bases: hence this section of the order is termed Basommatophora.
The bands of nerve-fibres uniting the various ganglia are termed 'commissures' when they unite the ganglia of the same pair, e.g. the cerebral; 'connectives' when they unite ganglia of different pairs, e.g. cerebro-pedal connective between the cerebral and pedal ganglia of the same side. The nervous system of H. pomatia has been worked out in detail by Bohmig. Each cerebral ganglion is divisible into three regions: from one there arise the commissure to the other cerebral ganglion the various connectives, and six nerves, one to the upper tentacle, the mouth, lip, pharynx, lower tentacle, and on the right side to the penis: from the second arises a nerve to the upper tentacle and eye: from the third nothing. The two pedal ganglia are each divisible into two regions, from which 8-9 nerves pass to the foot, and from one of the ganglia branches proceed to the connective tissue surrounding the uterus. The parieto-splanchnic region consists of a right and left pleural (=commissural) ganglion which are in union with the connectives to the cerebral and pedal ganglia, and with the remaining ganglia of the visceral system. These are a right and left visceral ganglion and a median abdominal.
The right visceral ganglion gives origin to two pallial nerves inclosed in a common sheath; the left to a single pallial nerve. The abdominal ganglion gives origin to three nerves; one distributed to the heart, nephridium, liver and (?) sexual organs; another to the neighbourhood of the anus, and the third to the integument. The buccal ganglia supply the pharynx, intestine and salivary glands with four nerves. Although the divisions between the pedal and the various ganglia of the visceral system are not visible externally in Helix and other terrestrial Pulmonata as they are in the aquatic Pulmonate Limnaeus, several facts may be noted which clearly show without further analysis the compound nature of the infra-oesophageal ganglion. These are (1) the distribution of the nerves given off by it; (2) the passage of the aorta cephalica through its centre; (3) the relation of the otocysts to the region termed pedal; (4) the presence of double connectives on each side between it and the cerebral ganglia, which can be clearly discerned through the common connective tissue sheath.
In Zonites algirus two pedal nerves run backwards in the foot nearly parallel one with another. They are connected by transverse commissures from place to place, and posteriorly they break up into a number of branches which anastomose with one another. Minute ganglia occur at the nodal points of this network, but are not common in the course of the two main nerves. The nerves to the margins of the foot originating from the pedal nerves form a similar network with nodal ganglia. In Vaginulus, according to Semper, the pedal nerves run parallel to one another, and are provided at stated intervals with ganglia from which spring transverse commissures. The similar transverse commissures of Limax appear from Simroth's description to be rather irregular, and they give origin to a network of fibres with nodal ganglia which unite them together. The nerves passing across the foot in Helix anastomose, but they are very irregular in arrangement; and in Arion they break up into a fine network, and there is nothing to compare with the commissures of Limax either in the direction or in the size of the nerves; but the networks of nerve-branches are furnished as usual with nodal ganglia both in Helix and Arion. The regular arrangements of transverse commissures in Zonites and Vaginulus recall the ladder-like structure of the pedal nervous system in Chiton, Haliotis and Fissurella among marine Gastropoda. It is not certain, however, whether or no they are strictly comparable with one another.