xii PHYLUM MOLLUSCA r,s:, 



Position of tlic Example. 



Triton nodiferus is one of several species of the genus 

 which is the only member of the family Tritonidm, belonging to 

 the sub-order Platypoda. The family Tritonidse differs from 

 the other families of the sub-order in the possession of a pro- 

 boscis, of a well-developed, but not greatly elongated, siphon, and 

 of a short foot. 







3. GENERAL ORGANISATION. 



External Features, Symmetry, &c. All the Gastropoda 

 are to a greater or less extent asymmetrical. In the young animal 

 the mouth is situated at the anterior and the anus at the posterior 

 extremity of the body. But, as a result of one-sided growth, a 

 great distortion of the parts takes place, leading to a more or less 

 pronounced asymmetry. Usually it is the left side which grows a 

 good deal more rapidly than the right. On the right-hand side 

 the space between mouth and anus increases relatively little, while 

 the left side develops rapidly. The result is the shifting of the 

 posterior parts the anus, the ctenidia, and the nephridial openings, 

 together with the mantle- cavity in which these are enclosed over 

 to the right side of the body, so that those parts come to be situ- 

 ated sometimes far forwards oil that side, sometimes close behind, 

 sometimes even above (dorsal to) the mouth. Sometimes the 

 ctenidia, originally situated to the right and left of the anus, 

 retain this arrangement after the anus has become displaced for- 

 wards, only that the originally right ctenidium is now on the left 

 and the originally left on the right ; 

 but in a large number of Gastropoda 

 the originally light; ctenidium aborts; 

 and, as will be more particularly de- 

 scribed later on, the same holds good 

 of the nephridia and their apertures. 

 This displacement of the anus takes 

 place pari passu with the great de- 

 velopment of the foot, and at the same 

 time with the outgrowth of the dorsal 

 region of the body into a great promin- 

 ence the visceral prominence con- 

 sequent on the great enlargement of p IG . 5S; >. shell of solarium per 

 certain of the internal organs, more s T.?FheS"J w^/ 



especially the liver and reproductive History.) 



organs. This visceral prominence is 



asymmetrically developed, projecting towards the right side; some- 

 times, as in the limpets (Patella), it is simply conical ; usually it 

 is coiled into a spiral of few or many turns enclosed in the shell. 



