THE TROCHOPHORE STAGE AS A FREE-SWIMMING LARVA. 



35 



compare this with the formation of the mantle in Cyclas at a later 

 stage (p. 43). 



The shell, in the condition just described, is already a real pro- 

 tection to the larva, for, on account of the contractility of the velum, 

 the whole body can be withdrawn between the two valves. The 

 shell increases in size as the larva grows ; in Dreixsensia concentric 

 bands of growth can soon be recognised, their number increasing 

 more and more with age. The growth of the larva of Dreistenoia, 

 and also that of the larvae 

 of marine Lamellibranchs 



before metamorphosis, is & 



very considerable. 



It need hardly be speci- 

 ally pointed out that the 

 'froehophnreot the Lamel- 

 libranchia and of the 

 Mollusca in general is, by 

 the possession of a shell, 

 distinguished in a very 

 noteworthy manner from 

 the Tn>i'//o/>/io/'''. larva of 

 other groups. Thus we 

 see that, in spite of all 

 the important points of 

 agreement, differentiation 

 in a special, and, for the 

 Mollusca, characteristic 

 direction, takes place at 

 this early stage. Other 

 Mnlluscan characters 

 affecting the body of the 

 Tror/tophore externally, 

 are the foot which arises 

 as an outgrowth of the 



body- wall between the mouth and the anus, and the gill-rudi- 

 ments, which are first indicated by papilla- or ridge-like outgrowths 

 of the ectoderm : but these changes will be dealt with later when 

 considering the transformation of the larva into the adult. Before 

 entering upon this subject, we have to describe an important char- 

 acter of the Trochophore larva itself, which marks still more strongly 

 its resemblance to the Annelidan Trochophore. 



FIG. 17. A-C, larvae of Dreissensia polyiiiorpfia in 

 various positions: A , surface view of the velum ; 

 B, antero- ventral aspect ; (older larva), seen 

 from the side (original) ; in, oral region ; s, shell. 

 The velum, especially in A, appears strongly pig- 

 mented. In (7, retractors are faintly seen running 

 back from the velum. 



