MOLLUSC A. 



215 



marine forms (e.g. Ostrea) the central flagellum is absent. It has been 

 suggested by LoveX though without any direct evidence, that the 

 labial tentacles of adult Lamellibranchiata are the remains of the 

 velum. The velar area is in any case the only representative of the 

 head. In some marine forms a general covering of cilia arises before 

 the formation of 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 elon- 

 gated 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 Lan- 

 kester, 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 re- 

 placed 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 ante- 

 rior, e.g. Cardium. 



The foot arises in 

 the usual way as a 

 prominence between 

 the mouth and anus. 

 In comparison with 

 Gasteropoda it is late 

 in appearing, and in 

 many cases does not 

 become prominent till 

 the shell has attained 

 a considerable size. In 

 its hinder part a pro- 

 visional paired byssus- 

 gland is developed from 

 the epidermis in Cy- 

 clas and other forms. 

 In other cases, e.g. My- 



FlG. 118. AN EMBRYO OF PlSIDIUM PUSILLUM. (FrOlll 



Lankester. ) 



/. foot; in. mouth; pli. pharynx; gs. bilobed 

 stomach; pi. intestine; ahs. shell-gland. 



