96 



SOLENOCONCHA. 



and ventral fusion of the shell-plate (Fig. 39 A and />), are retained 

 throughout life. 



The shape of the shell, which is at first cylindrical and then tusk- 

 like, is due to the mantle first assuming this form. The latter has 

 already been mentioned as growing out, like the shell, from the back 

 in the form of two folds, which fuse ventrally. Like the shell also 

 it remains open anteriorly and posteriorly. Anteriorly it grows to- 

 gether with the shell in the form of a tube for some distance over 

 the body which lies entirely hidden within it. The foot which, as 



B. 



I s. 



% 



--r"' / 







— - d. 



Fig. 39. — A, a larva of Dentalium undergoing metamorphosis ; I',, anterior portion of 

 a young Dentalium (after Lacaze-Duthiers). d, intestinal canal;/, foot; moe, 

 posterior aperture of the mantle ; •-;, shell ; /, tentacle-rudiment ; v, velum. 



we saw, originated as a large ventral swelling behind the oral aper- 

 ture, can be extended for some distance beyond the anterior aperture 

 of the mantle. It soon assumes the triangular form characteristic of 

 Dentalium (Fig. 39 .4 and B, /). In spite of the early development 

 of this exceedingly characteristic shape, it is not to be considered 

 a primitive featui'e, but must be regarded rather as a later acquisi- 

 tion, as it is wanting in a few genera Plate (No. 3). In Siphono- 

 dentalium and Gadulus the two lateral lobes are wanting, these genera 

 apparently exhibiting a more primitive form of foot. 



