MOLLUSC A. 213 



marked as that of the Lamellibranchiata, indicates that it is a 

 primitive phylum of the Odontophora. 



Scaphopoda, The external characters of the peculiar larva of this 

 interesting group have been fully worked out by Lacaze Duthiers 

 (No. 286). 



The segmentation is unequal and conforms to the usual molluscan 

 type. At its close the embryo becomes somewhat elongated, and 

 there appears on its surface a series of transverse ciliated rings. As 

 soon as these become formed the larva is hatched, and swims about 

 by means of its cilia. Six ciliated bands are formed in all, and in 

 addition a tuft of cilia is formed in a depression at the anterior 

 extremity. 



The larva thus constituted is very different in appearance to the 

 larvas already described, and its parts very difficult to identify ; the 

 next stages in the development shew however that the whole region of 

 the body taken up by the ciliated rings is part of the velar area, 

 while the small papilliform region behind is the post-velar part of the 

 embryo. This latter part grows rapidly, and at the same time the 

 ciliated rings become reduced to four; which gradually approach each 

 other, while the region on which they are placed grows in diameter. 

 The rings finally unite, and form a single ring on a projecting velar 

 ridge. In the centre of this ring is placed the terminal tuft of cilia 

 on a much reduced prominence. 



By the time that these changes have been effected in the velum, 

 the post-velar part of the embryo has become by far the largest 

 section of the embryo, so that the velum forms a projecting disc 

 at the front end of an elongated body. The mantle is formed as 

 two lateral outgrowths near the hinder extremity of the body which 

 leave between them a ventral groove lined by cilia; on their dorsal 

 side is formed a delicate shell. The mantle lobes continue to grow, 



5 



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 complete 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 Iarva3, 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 be- 

 yond 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 Dentalium. 



