THE DEVELOPMENT OF THE FORM OF THE LARVA. 



91 



become more conical, and the posterior (post-oral) part somewhat 

 lengthened (Fig. 36 A}. The blastopore, which has now become 

 narrow and slit-like, is displaced inwards by the development of an 

 ectodermal depression, the stomodaeum, which gives rise to the 

 buccal mass and the external aperture of which persists as the adult 

 mouth (Fig. 36 A}. The early larval stages of Dentolium closely 

 resemble those of Patella, as may be seen by comparing Figs. 35 and 

 36 with Fig. 50, p. 124. 



If we were justified in comparing the larvae of the Amphineura 

 and of the Lamellibranchia with the Annelidan Trochophore (pp. 

 5, 32 and 128), we may also attempt a similar comparison for the 



fl. 



V\c,. 'M. . I and It, median longitudinal sections through larvae of Di'-ntalium aged 

 respectively about 14 and 34 hours (after KOWALEVSKY). m, mouth ; md, enteron ; 

 /m'fi, mesoderm ; oes, stomodaeum ; sd, shell-gland ; w, ciliated ring ; tes, ciliated 

 tuft at the cephalic pole. 



larva of Ih'iitt ilium. In spite of the fact that the neural plate and 

 the kidney, two important organs of the Trocho/thore, have not as 

 yet been demonstrated in the larva of Dentalimn^ we can still see a 

 very striking resemblance to the Trochophore. Thus, in the conical 

 pre-oral region with its apical tuft of cilia, in the pre-oral ciliated 

 ring, in the relation of the blastopore to the future mouth, and in 

 the development of the body by an elongation of the post-oral region, 

 we see distinct Trochophoran characters. The anus only appears at 

 a later stage together with the paired rudiment of the cerebral 

 ganglion (which is perhaps connected with the cephalic plate). The 

 larva of Deutnliinn, however, may be distinguished by the presence 



