MOLLUSCA. 257 



early branched off from the main group of the Odontophora 

 along a special line of its own, and not that the remaining Odon- 

 tophora are descended from Chiton-like ancestral forms. The 

 shell of Mollusca on this view is not to be derived from one of 

 the plates of Chiton, but the plates of Chiton are to be derived 

 from the segmentation of a primitive simple shell. The segmen- 

 tation exhibited is of a kind which all the trochosphere larval 

 forms seem to have been capable of acquiring. The bilateral 

 symmetry of Chiton, which is quite as well 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 larvae 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 project- 

 ing 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 



B. II. 17 



