The capsule contains ciliated cells, and in some instances, at any rate, sense-cells. The calcareous otolith is either laminated and globular, or, as is most usual, consists of a mass of crystals.
1 Carriere in his account (Sehorgane der Thiere, 1885) denies the existence of a separate lens in the eye, and thinks that when such a lens-like structure is traceable it is a differentiation of the vitreous body and nothing more. Hilger states that the lens stains only peripherally with such a re-agent as haematoxylin, but the vitreous body throughout its substance. The embryonal lens of Paludina is laminated concentrically, and finely striated in a radial direction. Carriere's account of the retina differs from that of Hilger's mainly in the fact that he regards the non-pigmented cells as secretory and forming the vitreous body.
The clear cells (supra) evidently correspond to Patten's retinophorae, the pigmented to his retinulae. The former cannot therefore be considered, as they are by Fraisse and Carriere, as supporting or secretory cells. Patten has investigated only the eye of Haliotis, and points out its strong resemblance to the invaginated eye of Arca. See under Lamellibranchiata. He finds typical retinophorae and retinulae present. The latter bear rods; their basal ends are long slender hyaline stalks, or bacilli. The rods of both kinds of cells pass over into the vitreous body which fills the optic cup, and is a clear nerveless fluid. Its outer part is dense and forms the lens. The vitreous body and lens represent the corneal layer of the cuticula. At the margin of the optic cup they pass together with the retinidial layer by gradual transition into the cuticula of the hypodermis. See Mitth. Zool. Stat. Naples, vi. pp. 614-19; and discussion, pp. 619-23; and for the explanation of terms, the note, p. 452 of this book.
The head is always very distinct, and bears one or two pairs of tentacles, which are invaginable in the Stylommatophorous Ptdmonata. It is usually retractile, but there are exceptions, e.g. Patella. The buccal cavity is often armed with jaws, either a chitinoid plate on its upper wall, e.g. terrestrial Pultnonata, or two, one on each side, opposed to one another, varying in shape and consistency, and best developed in Azygobranchia. The mouth-bearing region of the head is greatly elongated in the carnivorous Azygobranchia, e.g. the Whelk, forming a proboscis, which can be retracted within a sheath. In other Azygobranchia the mouth may be placed at the end of a non-retractile snout known as rostrum. The sac of the radula varies much in size, and is greatly developed in the Limpet (Patella). The radula itself is occasionally wanting, e.g. Tethys (Non-Palliata). The form and arrangement of its teeth vary considerably, and have been employed as classificatory characters (see note, p. 116). Salivary glands are usually present, to the number of one or two pairs, opening into the buccal cavity. Their shape and size are extremely variable. In Dolium and its allies among the carnivorous Azygobranchia the salivary secretion contains free sulphuric acid.
Goblet cells are found in the buccal epithelium, sometimes collected into small groups. The oesophagus is often dilated into a crop, or has a lateral caecal crop attached to it (Lim-naeus, Planorbis, Buccinum). The stomach is usually simple in form, but in some palliate Opisthobranchia it is constricted into three or four portions, which differ from one another in internal characters, and in Non-Palliata it is often prolonged as a single or double caecum behind the pylorus. A small pyloric caecum is found also in other instances. The intestine is usually long, and coiled among the lobes of the liver, but is simple and much shortened in Non-Palliata. Its termination is sometimes dilated. The anus lies beneath the mantle fold, either on the right side or anteriorly. In Non-Palliata it is sometimes on the right side, but usually in or near the middle dorsal line, as e.g. in Doris. The stomach receives the ducts of the two branched glands known as liver. They are usually large, and often asymmetrical in point of size, especially when the visceral dome is large and spirally twisted.
In some Opisthobranchia the liver is represented by a variable number of lobes, which open into a caecum extending backwards from the stomach, or by a number of simple or branched processes originating from the stomach and its single, e.g. Eolis, or double, e.g. Antiopa, caecum. These simple and branched processes extend, one into each of the cerata (infra) in the Eolidia, and in many instances the food passes into them 'as it does in Scorpions' (Lankester). It is also said that they open at their apices to the exterior, at least in some instances. A caecal gland or 'pancreas' is said to exist in some Opisthobranchia, e.g. Doris, Aplysia, opening into the stomach. An anal gland opens into the anus in Purpura and Murex among Azygobranchia.
The heart consists of a thick-walled ventricle and thin-walled auricle, the latter receiving the blood from the respiratory organs. The ventricle is pierced by the intestine, and has a double auricle in Haliotis and Fissurella1. The ventricle gives origin to an aorta, which soon divides into a cephalic and an intestinal branch. The degree to which these two vessels are developed is very variable. The former may be prolonged to the head, passing between the pedal and pleural ganglia (Natantia, Pulmonata), and the latter may form a rich network over the intestine and liver, as in the Pulmonata. Eventually the blood passes into the lacunar system of the coelome, but the size of the lacunae is very variable, and in the Pulmonata some of them may have a more or less vessel-like character. The blood passes to and from the respiratory organs in special channels, and the character of the efferent sinuses depends upon the mode in which those organs are disposed, e. g. in many ceratonotous Non-Palliata there is a well-developed longitudinal sinus on each side the body, into which the blood returning from the cerata is collected.