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 



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

 ep. epidermis ; /. lens ; op. n. optic nerve ; r. retina ; v. h. vitreous humour. (From 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 

 ctenidium. In many of the 

 branchiate Streptoneura (Fig. 

 649), as already mentioned 

 in the case of Triton (see 

 p. 706, Fig. 623), the primi- 

 tively right osphradium, 

 which is alone developed, 



FIG. 649 Transverse section of osphradium of j,^^,^^. ft.~ f nrrn n f p npp 

 BXurex. br. n. branch nerve passing to lamina ; assumes tne L a pet 



*A?ter a sSyi Sphr ' n ' main osphradial nerve> tinate body with a central 



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 



