258 LAMELLIBRANCHIATA. 



cilia ; on their dorsal side is formed a delicate shell. The 

 mantle lobes continue to grow, and by the time the above 

 changes in the velum are effected they meet and unite in the 

 ventral line and convert the groove between them into a com- 

 plete tube open in front and behind. A stream of water is 

 driven through this tube by the action of the cilia. The shell, 

 which is at first disc-shaped like the shell of other molluscan 

 larvae, moulds itself upon the mantle and is so converted into a 

 tube. At the front end of the mantle tube, which does not at 

 first cover the velum, there is formed the foot. It arises as a 

 protuberance of the ventral wall of the body, which rapidly 

 grows forwards, becomes trilobed as in the adult, and ciliated. 



On the completion of these changes the larva mainly differs 

 in appearance from the adult by the projection of the velum 

 beyond the edge of the shell. The velum soon however begins 

 to atrophy ; and the larva sinks to the bottom. The mantle tube 

 and shell grow forward and completely envelop the velum, 

 which shortly afterwards disappears. The mouth is formed on 

 the ventral side of the velum at the base of the foot ; at its sides 

 arise the peculiar tentacles so characteristic of the adult Denta- 

 lium. 



LAMELLIBRANCHIATA. 



The larvae of Lamellibranchiata have in a general way the 

 same characters as those of Gasteropods and Pteropods. A 

 trochosphere stage with a velum but without a shell is succeeded 

 by a veliger stage with a still more developed velum, a dorsal 

 shell, and a ventral foot. 



The segmentation is unequal, and in a general way like that 

 of Gasteropoda, but the specially characteristic Gasteropodan 

 type with four large yolk spheres is only known to occur in 

 Pisidium, and a type of segmentation similar to that of Anodon 

 (p. 100) appears to be the most frequent. 



There is an epibolic or embolic gastrula, but the further 

 history of the formation of the germinal layers has been worked 

 out so imperfectly, and for so few types, that it is not possible to 

 make general statements about it. What is known on this head 



