XII 



PHYLUM MOLLUSCA 



747 



primitively right osphradium, which is alone developed, assumes 

 the form of a pectinate body with a central ridge, on either side 



FIG. 638. Eyes of Gastropoda. A, Patella; B, Trochus ; C, Turbo; D, Murex. 

 cj>. epidermis; I. lens; op. n. optic nerve; /. retina; v. h. vitreous humour. (From the 

 Cambridge Natural History, after Helger.) 



of which is a row of close-set lateral lamina?, and is commonly 

 termed the parcibranckia from its resemblance in appearance to 

 a gill. In some cases it is of even more complicated shape than in 

 Triton, owing to the branch- 

 ing of the lateral ridges. 



The nephridia of the 

 Gastropoda are dorsally 

 placed glandular tubes or 

 chambers, which communi- 

 cate internally with the peri- 

 cardium, and open on the 

 exterior, either directly or 

 through a duct the ureter. 

 Both right and left neph- 

 ridia may be present, though unequal in size, the one situated 

 to the right of the anus being larger than that situated to 

 the left : or the former may alone be developed (Euthyneura). 



FIG. 639. Transverse section of osphradium of 

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

 I a <m. laminae; osphr. n. main ospbradial nerve. 

 (After Spengel.) 



