GASTROPODA ANISOPLEURA. 477 



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 

 (Ptdmonata), 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. 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. 



The head is always very distinct, and bears one or two pairs of 

 tentacles, which are invaginable in the Stylommatophorous Pulmonata. 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 Pulmonata, 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 carni- 

 vorous 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, Bticcinum). 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 



