260 



LAMELLIBRANCHIATA. 



the velum ; and in Montacuta and other types there is developed, 

 as in many Gasteropoda, a circum-anal patch of cilia. 



A shell-gland appears at a very early period on the dorsal 

 surface in Pisidium, Cyclas and Ostrea, and probably in most 

 marine forms (fig. 118, shs}. It is somewhat saddle-shaped, and 

 formed of elongated non-ciliated cells bounding a groove. It 

 flattens out and on its surface is formed the shell, which appears 

 usually to have the form of an unpaired saddle-shaped cuticle, on 

 the two sides of which the valves are subsequently formed by a 

 deposit of calcareous salts. In Pisidium the two valves are 

 stated by Lankester to be at first quite independent and widely 

 separated, and it has been suggested by Lankester, though not 

 proved, that the ligament of the shell is developed in the median 

 part of the groove of the shell-gland. 



The mantle lobes are developed as lateral outgrowths of the 

 body : they usually have a considerable extension before they 

 are covered by the shell. In Anodon and Unio the larval 

 mantle lobes are, however, formed in a somewhat exceptional 

 way, and are from the first completely covered by the valves of 

 the larval shell. The larval mantle lobes and shell in Anodon 

 and Unio are subsequently replaced by the permanent structures. 



The adductor muscles are formed soon after the appearance 

 of the shell. The 

 posterior sometimes 

 appears first, e.g. 

 Mytilus, and at other 

 times the anterior, 

 e.g. Cardium. 



The foot arises 

 in the usual way as 

 a prominence be- 

 tween the mouth 

 and anus. In com- 

 parison with Gaster- 

 opoda it is late in 

 appearing, and in 

 many cases does not 

 become prominent 

 till the shell has at- 



FlG. Il8. AN EMBRYO OF PlSIDiUM PUSILLUM. 

 (From Lankester.) 



f. foot ; m. mouth ; ph. pharynx ; gs. bilobed stomach ; 

 pi. intestine ; shs. shell-gland. 



