122 DESCRIPTIONS OF PREPARATIONS. 



overlying epidermic cells have also a very thick cuticula. A tentacular ganglion is 

 found on the outer side near the bases of the two tentacles of Basommatophora, 

 and an oral ganglion in the oral lobes, e. g. of Limnaeus. 



The eyes are situated close to the apex of the upper tentacles, a little to their 

 inner side. The optic nerve is a branch of the tentacular nerve in the Stylom- 

 matophora, an independent nerve in the Basommatophora. The retina is composed 

 of a single layer of cells derived by invagination from the epidermis. The cells are 

 of two kinds, pigmented and non-pigmented, both continuous basally with the 

 nerve-fibrils derived from a peripheral optic ganglion. The pigmented cells are 

 widest at their inner, the non-pigmented at their outer ends. The inner ends of the 

 non-pigmented cells end in a flask-shaped (Stylommatophora) or a fine (Basom- 

 matophora} visual rod. The inner ends of the pigmented cells form transparent 

 processes. Each non-pigmented cell is surrounded by a zone of pigmented, and 

 the visual rods are encased in the transparent processes of the latter. The retina is 

 continuous at its anterior margin with a layer of transparent cells which lines the 

 cornea (=pellucida). The optic cavity is filled by a structureless substance, the 

 vitreous body (Carriere), commonly called the lens. By Hilger a vitreous body is 

 distinguished from a lens in all the Gastropoda he examined except the Stylom- 

 matophora, which, according to him, possess a lens but no vitreous body. It is 

 probable that the lens, so called, is a differentiated portion of the vitreous body, 

 and not an independently formed structure. The cornea consists of an outer layer 

 of transparent cells with underlying connective tissue. 



The otocyst is small in size, and is to be found on the pedal ganglion close to 

 the spot where the anterior bundle of pedal nerves quits it. To the naked eye it 

 appears like a minute white dot. It consists of a connective tissue capsule nearly, 

 but not quite, spherical, and an epithelial lining with a central cavity. The latter 

 contains a fluid and a number of otoliths, as is usual in Gastropoda. The 

 otoliths are more or less oval, and consist of a small quantity of an organic 

 substance and a large quantity of calcium carbonate. The cells of the epithelium 

 are not clearly separated from each other in the fresh state, according to Leydig. 

 At the spot opposite to the auditory nerve (or the auditory canal) they are of 

 larger size. All the cells are ciliated, and the cilia are short. It is doubtful whether 

 or not long auditory or sensory hairs are present such as are found in Natantia 

 (=Heteropodd) and in Cydas (a Lamellibranch). Leydig believes them to be 

 present. The otoliths are in constant motion in the living animal. The otocyst is 

 connected to the cerebral ganglion by a delicate filament. This, according to 

 Leydig, as in some other Gastropoda, is a narrow canal lined by cells, and continuous 

 with the cavity of the otocyst. Nerve-fibrillae are therefore not present. But if this 

 is the case, it is difficult to see in what way auditory impulses can affect the 

 nervous system. And, judging from the analogy of other Mollusca, auditory 

 nerve-fibrils are probably present. The connection of the otocysts with the pedal 

 ganglia is only accidental. Both sets of structures are enveloped by a common 

 investment of loose connective tissue. 



An osphradium or olfactory organ in connection with the organ of respiration 

 has not been detected in Helix pomatia and many of its allies. In Helix per sonata 

 Sarasin found a nerve arising from the region of the right visceral ganglion which 

 passes beneath the epithelium of the pulmonary chamber, the cells being more 



