296 INVERTEBRATE MORPHOLOGY. 



Gasteropods consequently a shell coiled in a right-handed 

 spiral occurs, but this rule has not a few exceptions. "Where 

 the shell forms a left-handed spiral it is to be explained by 

 supposing that in such cases the visceral hump tended towards 

 the right side of the body rather than the left, and this is 

 confirmed by the fact that in most left-handed forms it is the 

 left branchia and nephridium that have persisted. 



It must be pointed out, however, that the extent to which 

 the rotation of the mantle-cavity, the abortion of the organs 

 of either the left or right side of the body, and the crossing 

 of the pleuro-parietal nerve-cords has been carried varies in 

 different forms. In some the rotation has been carried so far 

 that the original right branchia, etc., has passed the median 

 line in front so as to lie on the left side of the body, and in 

 such cases the crossing of the nerve-cords (chiastoueurisrn) is 

 completed. Many forms, however, stop short of this, and 

 numerous gradations are to be found. The rotation, however, 

 is present in all forms to some extent and forms a character- 

 istic feature of Gasteropod morphology. 



The anterior portion of the body (Fig. 132) is usually well 

 marked off by a more or less distinct constriction or neck, and 

 consequently it is possible in the Gasteropods to speak of a 

 head in contradistinction to the trunk region of the body ; in- 

 deed so prevalent is this character that the term Cephalo- 

 pliora has been applied to the group. Tentacles, either one or 

 two pairs, are borne by the head, and furthermore eyes are 

 usually present upon it either at the bases of one of the pairs 

 of tentacles or else borne at the tips of these structures. 



The foot is generally well developed and usually has a flat 

 creeping sole. It undergoes many modifications, however, 

 sometimes becoming more keel-like, or becoming differentiated 

 into three regions differing in form, the propodium, mesopo- 

 diuni, and metapodium, the last-named portion frequently 

 secreting a chitinous plate, the operculuni (Fig. 132, op), which 

 serves to close the mouth of the shell when the animal is 

 withdrawn within it. In addition to these portions an epi po- 

 dium is frequently highly developed, consisting in its primi- 

 tive form of a fold arising from the sides of the foot where it 

 passes into the body-wall. In many cases, however, it loses 



