GASTROPODA. 



367 



supra-intestinal ganglion, or from the part of the visceral commissure 

 where these ganglia would be if present. In the Chitonidae there 

 is an osphradium at the base of each ctenidium. In Fissurella it 

 is only present in a diffuse and indistinct form. It is aljp found in 

 some forms in which the ctenidium is absent (Patella, Clione, etc., 

 Basommatophora). 



Otocysts which are supposed to be auditory in function exist in 

 most gastropods (absent in lanthina, Vermetus) as a pair of closed 

 sacs, which are lined by an epithelium bearing cilia and sensory 

 hairs, and contain concretions and a fluid. They are usually placed 

 in the foot near the pedal ganglia (though innervated from the 

 cerebral by way of the cerebro-pedal commissure, Fig. 283), but in the 

 pelagic forms (Heteropoda) and in most Nudibranchs they are near 

 the cerebral ganglia (Fig. 315). They contain a fluid, and one large 

 concretion the otolith (as in the Ctenobranchiate Streptoneura, and a 

 few Opisthobranchs), or 

 numerous small ones 

 the otoconia (Aspido- 

 branchs, most Euthy- 

 neura and dialyneuric 

 Taenioglossa). It has 

 riot been proved by 

 actual observation that 

 any gastropod possesses 

 the sense of hearing. 



Eyes. There is a pair 

 of cephalic eyes in almost 

 all gastropods. They are 

 placed at the base of the 

 tentacles (of the second 

 pair in Opisthobranchs) 

 or at the apex of the 

 posterior tentacles (Sty- 

 lommatophord), or half- 

 way along the tentacles 

 (some Streptoneurd). 

 They consist in their 

 simplest form of a widely open pit of the skin (Fig. 290 A), the 

 epithelium of which is pigmented and carries on its free surface 

 a layer of rods (Docoglossd) ; in some of the Rhipidoglossa, e.g., 

 Haliotis, Trochus, the lips of this pit are approximated and its 



JT .5 



FIG. 289. Otocyst of Pterotrachea (after Glaus). 1 auditory 

 nerve ; 3 structureless membrane ; 3 and k ciliated 

 cells ; 5 otolith ; 6 hair cells ; 8 central cell (from 

 Lang). 



