xn 



PHYLUM MOLLUSCA 



727 



in Oncidium is occupied by a refractive body composed of a 

 few large transparent cells. 



The statocysts are usually placed in close relation to the pedal 

 ganglia, but are always innervated from the cerebral. An olfactory 



o^l I 



FIG. 648. Eyes of Gastropoda. A, Patella ; B. Trochus ; C, Turbo; D. Wlurcx. 



ep. epidermis ; 1. lens ; op. n. optic nerve ; r. retina ; v. h. vitreous humour, (i'rom the 

 Cambridge Natural History, after Helger.) 



organ is present in the shape of groups of cells on the tentacles, 

 in which the fibres of an olfactory nerve terminate. 



The osphradia are prominences, usually of simple form, situated 



close to the base of the 

 tct7n, ctenidium. In many of the 

 branchiate Streptoneura (Fig. 

 649), as already mentioned 

 in the case of Triton (see 

 LL P- 706, Fig. 623), the primi- 

 tively right osphradium, 

 which is alone developed, 



FIG. 649. Transverse section of osphradium of ne ,o 1lrnp a fU p f nrrn n f n T)PP- 



Murex. br. n. branch nerve passing to lamina ; &SS 



lam. laminae ; osjihr. n. main osphradial nerve, tiuate body with a Central 



(After Spengel.) ... .', . ,. , 



ridge, on either side of which 



is a row of close-set lateral laminae, and is commonly termed the 

 parabranchia from its resemblance in appearance to a gill. In 



