476 THE ANIMAL KINGDOM. 



tentacles, and ganglion cells beneath the lateral organs. A sensory organ 

 known as osphradium lies near the ctenidium. It is present in the Limpet 

 (Patella), and in some Pulmonata, e.g. Limnaeus, where the ctenidium is 

 aborted. It consists of a patch of cylindrical ciliated cells, with an under- 

 lying ganglion, and is always supplied with a nerve from the visceral loop 

 or ganglion. There are two osphradia in the Zygobranchia, one right, the 

 other left. Gustatory buds have been detected in the oral cavity of 

 Fissurella and Trochus. The eyes are usually two in number, situated one 

 at the base of each cephalic tentacle, and sometimes raised on a papilla. 

 In certain Pulmonata, hence Stylommatophora, they are placed at the tips 

 of the two superior tentacles. In some Non-Palliata, e.g. Eolis, Doris, and 

 the palliate Opisthobranch Philine, they lie upon the cerebral ganglia, and 

 are then small in size. In Patella the eye is a cup-shaped depression, 

 widely open ; in Haliotis, some species of Trochus , nearly closed ; in other 

 Anisopleura quite closed. The retina is formed of a single layer of cells, 

 differentiated into pigmented and non-pigmented. The latter are broadest 

 at their bases, narrow at their inner ends, which are prolonged into a delicate 

 rod. The former are broadest at their inner ends, which are furnished with 

 rods, narrow at their bases, and they surround, in groups containing four to 

 eight cells, each non-pigmented cell, the visual rod of which is enveloped 

 by their rods. Both (?) kinds of cells are connected by basal processes 

 with the fibres of the optic nerve, which spread over the back of the eye, 

 and in many instances at least contain intercalated ganglion cells. In 

 Helix there is a well-developed peripheral optic ganglion (Carriere). The 

 open retinal capsule contains a vitreous body in Haliotis, (? Patella] ; the 

 closed a vitreous body and a lens, which occupies the anterior part of the 

 capsule. The former is soft, the latter more dense and slightly yellow ; 

 both are structureless. A vitreous body is said to be absent in stylom- 

 matophorous Pulmonata (Hilger and others) 1 . The internally-placed eye 



1 Carribre 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 sup- 

 porting or secretory cells. Patten has investigated only the eye of Haliotis, and points out its strong 

 resemblance to the invaginated eye of Area. 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. 



