THE TRANSFORMATION INTO THE ADULT. 41 



to demonstrate the presence of the mesoderm-bands, the shell-gland 

 and the neural plate. 



The organisation of the embryos of fresh-water Lamellibranchs 

 just described renders it indisputable that they represent the Trorlm- 

 tfkore stage. As in Gyclas, so in Pisidmm this point can be proved ; 

 in the Unionidce, indeed, the modification has been greater, and it is 

 therefore very difficult to recognise in them the organisation of the 

 Trochophore. Even here, however, there is a remains of the ciliated 

 apparatus (Figs. 22-24, p. 53) which causes the well-known rotation 

 • >f the embryo within the egg-integument, but this ciliated area, 

 according to the definite accounts of Schierholz, Schmidt and 

 Goette, does not lie anteriorly, but in the posterior part of the 

 body, so that it cannot here be considered as a vestige of the velum, 

 as some have attempted to maintain, but rather as corresponding to 

 the ciliated anal tuft. 



4. The Transformation into the Adult. 



It will be seen from the foregoing account that the presence of a 

 free-swimming larva is to be regarded as an indication of a primitive 

 condition in the Lamellibranchia. We should consequently expect 

 to find in those forms that possess this ontogenetic stage that the 

 changes which bring about the transformation of this free larva into 

 the adult would also be of a primitive character. But, unfortunately, 

 the whole of the further development is not known in the case of any 

 marine Lamellibranch, so that we are obliged to confine ourselves- 

 chiefly to the fresh-water Lamellibranchia in discussing this subject, 

 although, as we have seen, we must, for the most part, regard 

 them as modified forms. In Cycfas, however, among the latter, the 

 Trochophore stage is distinctly developed, and we are therefore per- 

 haps justified in assuming that the process of metamorphosis has, 

 in this case, not undergone any very great modification. For 

 purposes of comparison, we shall avail ourselves of the few data which 

 have been obtained relating to the development of the marine 

 Lamellibranchs. 



The mantle develops as early as the Trochophore stage in the 

 marine Lamellibranchia, and, with the shell, surrounds a large part 

 of the body. On each side, the mantle-folds are separated by a narrow 

 but deep fissure-like cavity from the keel-shaped body ( Teredo). The 

 foot is not to be seen at this stage (Fig. 18) ; it arises at a later stage 



