b AMPHINEURA. 



proctodaeum (Fig. 9). An organ which is of great importance in interpreting 

 the larva, the apical plate, is not present in the early stages of Chiton, but 

 the cerebral ganglia arise later in the position which this organ occupies in 

 the Annelida ; in the free-swimming larva of Chiton Polii these ganglia are 

 found beneath the ciliated tuft on the frontal pole (Fig. 5.), and may therefore 

 be regarded as representing the apical plate. Thus, to make the comparison 

 complete, only the primitive kidneys are wanting. So far these have not 

 been found, although they occur in other Molluscs (pp. 39 and 136). 



a. 



<J3. 



Fig. 3. — .4, larva of Chiton marginatus (after Loven) ; B, embryo ; aud C, larva of 

 C. Polii (after Kowalevsky) ; a, eye ; k, rudiments of shell-plates ; m, mouth : 

 st, spines; w, ciliated ring (velum); ww, apical ciliated tuft. 



3. The Further Development of the Larva and Metamorphosis. 



The changes that now take place in the larva are introduced by 

 considerable growth of the posterior region of the body (Figs. 2 and 5), 

 a process which recalls the development of this same section in the 

 Annelidan Trochophore into the trunk region of the worm (Vol. i., 

 Fig. 120, p. 269 ; cf. also Chapter xxxiii.j. In the case of Chiton, 

 it is especially the dorsal surface which increases in size (Fig. 5). 

 In some cases, these changes take place even in the embryo, and as 



