270 SUMMARY. 



General review of the characters of the Molluscan larva. 



The typical larva of a Mollusc, as has been more especially 

 pointed out by Lankester, is essentially similar to the larva of a 

 number of invertebrate types, and especially the Chaetopoda, 

 with the addition of certain special organs characteristic of the 

 Mollusca. 



It has a bent alimentary tract, with a mouth on the ventral 

 surface and a terminal or ventral anus. The alimentary tract is 

 divided into three regions : oesophagus, stomach, and intestine. 

 There is a variously developed praeoral lobe with a ring of cilia 

 the velum, and a perianal lobe, often with a patch of cilia 

 (Paludina, etc.). In all these characters it is essentially similar 

 to a Chaetopod larva. The two characteristic molluscan organs 

 are (i) a foot between the mouth and anus, and (2) an in- 

 vagination of the epiblast on the dorsal side at the hinder 

 end of the body, which is connected with the formation of the 

 shell. 



The larvae of most Gasteropoda, Pteropoda, and Lamelli- 

 branchiata present no features which call for special remark ; 

 but the larvae of the Gymnosomata amongst the Pteropoda, and 

 of the Scaphopoda, Polyplacophora and Cephalopoda present 

 interesting peculiarities. 



The larvae of the Gymnosomata are peculiar in the presence 

 of three transverse ciliated rings, situated beJiind the velum (Fig. 

 109). These rings might be regarded as indications of a rudi- 

 mentary segmentation ; but, as already indicated, this view 

 is not satisfactory. There is every reason for thinking that 

 these rings have been specially acquired by these larvae. 



At first sight the larvae of the Gymnosomata might be 

 supposed to resemble those of the Scaphopoda, which are also 

 provided with transverse ciliated rings ; but, as shewn above, the 

 rings of the Scaphopoda are merely parts of the extended velar 

 ring. 



Thus, the ciliated rings of the two larvae so similar in 

 appearance are in reality structures of entirely different values, 

 being in the one case parts of the velum, and in the other special 

 developments of cilia behind the velum. 



