696 



ZOOLOGY 



SECT. 



organ is present in the shape of groups of cells, in which the 

 fibres of an olfactory nerve terminate, situated on the tentacles. 



FIG. 601. Eyes of Gastropoda. A, Patella; B, Trochus } C, Turbo; D, Murex. 

 ep. epidermis ; 1. lens ; op. n. optic nerve ; r. retina ; r. h. vitreous humour. (From the 

 Cambridge Natural History, after Helger.) 



The osphradia are prominences, usually of simple form, situated 

 close to the base of the ctenidium. In many of the branchiate 

 Streptoneura (Fig. 602), as already mentioned in the case of Triton 



(see p. 676, Fig. 576), the 

 right osphradium, which is 

 alone developed, assumes 

 the form of a pectinate 

 body with a central ridge, 

 on either side of which is 

 a row of close-set lateral 

 laminae, and is commonly 

 termed the parctbrancJiia 

 from its resemblance in 

 appearance to a gill. In 

 some cases it is of even more complicated shape than in Triton, 

 owing to the branching of the lateral ridges. 



FIG. 602. Transverse section of osphradium of 

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

 lam. laminae ; osphr. n. main, osphradial nerve. 

 (After Spengcl.) 



