THE TRANSFORMATION OF THE LARVA INTO THE ADULT. 



95 



3. The Transformation of the Larva into the Adult. 



Even at the time when the larva sinks to the ground, though still 

 at first moving with the help of the velum, the principal organs of 

 the adult are already present as rudiments. This last period of its 

 development is therefore marked by the growth and the further 

 development of rudiments already present in it. 



If we examine the larva externally (Fig. 38 B), we find that the 

 shell has grown much larger. At first it was a disc-like structure 

 lying on the back, but then it became saddle-shaped, growing down the 

 sides of the larva till its free edges united in the ventral middle line 

 38 A). In the 



A. 



B. 



ventral parts of the 

 shell, a parallel 

 striation can be recog- 

 nised (Fig. 38 B), 

 representing lines of 

 growth, so that the 

 growth takes place 

 here in the same way 

 as in the shells of the 

 Lamellibranchia (Fig. 

 27, p. 60). As the 

 shell increases in size, 

 the fusion of the 

 ventral margins be- 

 comes closer. At first 

 the anterior aperture of the shell is still considerably wider than the 

 posterior, a condition connected with the shape of the larva (Fig. 

 38 B), but when the velum degenerates and the shell lengthens, the 

 anterior aperture becomes relatively smaller. The shell now appears 

 almost cylindrical, its anterior aperture being somewhat wider than 

 its posterior aperture. Its increase in size is caused by the secretion 

 of new shell -material from the anterior tubular margin of the fused 

 mantle-folds, the newly formed parts being marked off from the older 

 parts by circular boundary lines ; these latter give the shell the 

 appearance, especially in older animals, of being segmented (Fig. 39). 

 At a later stage the shell assumes a dorsal curvature and gradually 

 acquires the tubular conical shape found in the adult. The anterior 

 and posterior apertures, which originated through the lateral growth 



FIG. 38. Larvae of Dentalium, A at the end of the 

 second day and tt on the third or fourth day. A, 

 seen from the ventral side, B, seen somewhat obliquely 

 from the same side (after LACAZE-DUTHIERS). /, foot ; 

 moe, posterior aperture of the mantle ; s, shell ; w, 

 ciliated ring (r, velum) ; ws, ciliated tuft. 



