704 



ZOOLOGY 



SECT. 



organs advance in development the velum decreases in size and 

 gradually aborts. 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 

 Onchidium, though the trochosphere 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 

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

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



lentt, 



ve.1 



B 



lent 



sh 



FIG. 610. Veliger stage of Vermetus. <-,,-. //. cerebral ganglia; ey. eye ; /. foot; mo. mouth; 

 ot. otoeyst ; sk. shell; le,it. tentacle ; -cd. velum. (After Lacaze-Duthiers.) 



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 to the 

 neighbourhood of the anterior end ; in the Streptoneura the 

 anus, etc., in their displacement forward pass beyond the middle 

 line, one of the most striking effects of which is the crossing of the 

 pleuro-visceral commissures, already referred to (p. 694). 



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 

 a reversed position, adhering to frothy mucus secreted by the 



