THE TROCHOPHOKE STAGE AS A FREE-SWIMMING LARVA. 36 



A 



B. 



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

 stage (p. 43). 



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

 tection to the larva, for, .»n account of the contractility of the velum, 

 the whole body can he withdrawn between the two valves.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

 shell increases in size as the larva -rows ; in Dreissensia concentric 

 bands of growth can soon be recognised, their number increasing 

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

 and also that of the larvae 

 of marine Lamellibranchs 

 before metamorphosis, is 

 very considerable. 



It need hardly be speci- 

 ally pointed out that the 

 Trochophoreof theLamel- 

 libranchia and of the 

 Mollusca in general is, by 



the possession of a shell, 



distinguished in a very 



noteworthy manner from 



the Trochophore 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 



Molluscan characters 



affecting the body of the 



Trochophore 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. 



' u; - 17- — A-G, larvae of Dreissensia polymorphs, in 



various positions: .1, surface view of the velum - 

 P>, antero-ventral aspect ; (older larva), seen' 

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

 The velum, especially in .1 , appears strongly pig- 

 mented. In/', retractors are faintly seen running 

 back from the velum. 



