

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 also found in 

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

 Basommatopliora). 



Otocysts which are supposed to be auditory in function exist in 

 most gastropods (absent in lantMna, 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 (Heteropocla) and in most Nudibranchs they are near 

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

 concretion the otolitli (as in the Ctenobranchiate Streptorieura, and a 

 few Opisthobranchs), or 

 numerous small ones 

 the otoconia (Aspido- 

 branchs, most Euthy- 



neura and dialyneuric 



Taenioglossa). It has 



not 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- 



lommatophora), or half- 

 way along the tentacles 



(some Streptoneura). 



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 



---& 



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

 nerve ; 3 structureless membrane ; 3 and k ciliated 

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

 Lang). 



