XII 



PHYLUM MOLLUSC A 



765 



the operculum. Two little processes on the velar area develop into 

 the tentacles (tent.), and the eyes (ey.) appear at their bases. As 

 the foot and other organs advance in development the velum 

 decreases in size and gradually aborts, but in some cases a portion 

 of it persists as the subtentacular lobes or labial tentacles in the 

 neighbourhood of the mouth. 



In the Pulmonata the velum is not well developed, except in 

 Oncidium, though the trochophore stage is well marked. 



The young Gastropod is at first bilaterally symmetrical ; the 

 prevailing asymmetry is the result of unequal growth of the two 

 sides of the body. In the majority of cases it is the left side that 

 grows more actively than the right, a result of which is that the 



Fio. 647.— Veliger stage of Vermetus. cer. g. cerebral ganglia ; ty. eye ; ). foot ; mo. mouth ; 

 ot. statocyst ; $h. shell ; tent, tentacle ; vel. velum. (After Lacaze-Duthiers.) 



posterior parts — the anus and the region surrounding it — are dis- 

 placed forwards towards the right, the space between the anus and 

 the mouth on that side undergoing little or no increase in length. 

 In the Opisthobranchia and the Pulmonata the anus with the 

 mantle-cavity and its contents become displaced forwards towards 

 the anterior end ; in most of the Streptoneura the anus, &c, in 

 their displacement forward pass beyond the middle line, one of the 

 most striking effects of which is the crossing of the pleuro-visceral 

 connectives already referred to (p. 737). 



Ethology and Distribution. — Only a few aberrant families of 

 Gastropoda are parasites. Most are aquatic, all the most primitive 

 forms being inhabitants of the sea. Of the marine families the 

 majority move by creeping over the sea-bottom, some burrowing 

 in mud or sand, some in solid rock; some are able to. float in 



