The body-wall consists of connective tissue and muscle-fibres. Its surface in the exposed parts of the body is raised into ridges and tubercles. The muscle-fibres are non-striated long cells. They occasionally appear striated, but the cause of the striation is not known. The connective tissue consists of plasma-cells, a matrix, and fibrils. The plasma-cells are richly developed in all Mollusca, but especially in Pulmonata where three kinds are distinguishable. These are: (1) oval or round in shape with a transparent protoplasm and a round nucleus; (2) irregular cells containing refractile granules which are not fatty in nature; and (3) cells with granules of lime carbonate, which are immeasurably fine in the interstitial tissue of the upper coils of the visceral dome, coarser elsewhere. These cells are imbedded singly or in masses in a matrix which contains stellate cells, and connective tissue fibrils, the latter much more scanty than in other Mollusca. The matrix is sometimes much reduced, and the first kind of plasma-cells then appears to form sheaths round various organs, e. g. nerves, but the stellate cells of the matrix may still be detected among them.
The connective tissue membranes are generally pierced by apertures, many of which are bounded by refractile rings, the product of several cells which encircle the aperture. Glycogen has been detected in the plasma-cells of the first kind both in Anodon and Helix (Blundstone, P. R. S. xxxviii. 1884-85). Branched pigment-cells are found in the cutis and sometimes extend inwards to its deeper layers. Changeable chromato blasts have been observed by Leydig in Limax variegatus and L. (=Amalia) carinatus (A. M. A. xii. 1876, p. 541).
The buccal mass consists chiefly of an odontophore, using that term in a wide sense to include the muscular and cartilaginous apparatus in connection with a chitinoid (?) radula or lingual membrane bearing transverse rows of teeth. The radula is developed within a radular sac and is perpetually growing throughout life. The sac is essentially a ventral diverticulum of the buccal cavity, and its lining cells are continuous with the oral epithelium. It is crescentic in transverse section. At the blind end of the sac the lining cells are differentiated into odontoblast cells, which form a transverse ridge broken up in correspondence with the number of teeth present. This ridge contains in vertical section four to five large cells in Pulmonata and Opisthobranchia; a number of elongated cells in other Glossophora. The singly refractile core of each tooth is secreted by these cells, the doubly refractile enamel-like outer layer by the cells lying immediately dorsal to the odontoblast cells, whilst the membrane or matrix uniting the bases of the teeth is formed by a single cell in Pulmonata and Opisthobranchia, by several in other Glossophora, placed in each case just ventral to the odontoblast cells. These matrix cells split up at their free ends into fibres.
The radula is carried by a subradular membrane, developed by the cells of the ventral wall of the sac. The membrane is borne upon cartilaginous pads, and the whole is worked by a system of muscles, protractor and retractor. Other muscles flatten the radula or convert it into a groove. And in some cases the subradular membrane with the radula slides backwards and forwards to a limited extent over its cartilaginous supports.
The chromogen, called myohaematin by MacMunn, is found in the buccal muscles (and in the heart) of Helix, Arion, Limax and of other Pulmonata. According to Ray Lankester, haemoglobin occurs in the buccal mass of some Mollusca.
The form and number of the teeth in a transverse row are very variable in the Glossophora. In Pulmonata a median tooth may be distinguished from an indefinite number of admedian teeth. Such a dentition may be formulated thus - ∞ 1 . ∞, and is termed myrioglossate. Various technical terms have been applied to the variations in number and arrangement of the teeth already alluded to. A median tooth may be present or absent: so too admedian teeth: and in the arrangement known as rhipidoglossate lateral may be distinguished from admedian and median teeth. The size and shape of the teeth themselves are also extremely variable. In the Pulmonata each tooth is provided with a recurved hook, usually simple, but sometimes denticulated1.
The more or less chitinoid jaw lies on the dorsal aspect of the oral cavity. In most Helices it is a crescentic plate; its free surface provided with anteroposterior ridges. The form of the jaw varies in different Pulmonata: in aquatic Pulmonata it may consist of more than one piece, and in a few terrestrial forms it is absent, e. g. Testacella.
The salivary glands vary in form and size in different Pulmonata. They are compound glands, but the ultimate acini are composed of a number of unicellular glands. They receive bloodvessels and nerves, the latter from the buccal ganglia. An additional salivary gland lies imbedded in the buccal mass in Helix pomatia round the entrance of the main salivary ducts. It consists of unicellular glands, partly opening into the salivary ducts, partly uniting and opening by ducts of their own into the oral cavity. The salivary extract converts starch into sugar. The muscular coats of the digestive tract are an external circular, and an internal longitudinal coat. The longitudinal ridges of the internal surface are formed by the longitudinal muscles. The epithelial cells possess a cuticle, and cilia are present in places; in young Helices over the whole surface of the stomach. Goblet cells are present in Zonites and apparently in Helix; and in the former Nalepa observed small cells at the base of the other cells, which they are apparently destined to replace. The liver or hepato-pancreas is a compound acinous gland. Its acini are held together by connective tissue, ramifying bloodvessels and nerves. The epithelium forms a single layer.