156 



INVERTEBRATA 



CHAP. 



and left daughters, and these begin to found the mesodermic bands, 

 Wilson has recently pointed out, however (1898), that 4d is 

 sufficiently reminiscent of its endodermic origin to contribute six 

 to ten small cells to the formation of the intestinal wall. 



When the larva of Nereis is fully developed, i.e. when the alimen- 

 tary canal has become functional, it is not only provided with a post- 

 troohal worm-body but this body shows the rudiments of no less than 

 three pairs of parapodia, and this seems to be a general feature 

 amongst many Polychaeta. Further, behind each pair of parapodia 

 is an accessory ring of cilia, so that in Nereis we have a typical 

 example of a Polytrochal larva. In the apical region there is to be 



seen a row of five 

 gland cells — these 

 seem to be homologous 

 with the gland cells in 

 the upper hemisphere 

 of the Trochophore of 

 Polygordia. 



The development 

 of the genus Gapitella 

 has been worked out 

 in great detail by 

 Eisig (1900). His 

 results are chiefly re- 

 markable for his asser- 

 tion that, in this form, 

 the mother cells of the 

 adult mesoderm arise 

 from the third and not 

 the fourth quartette of 

 micromeres. We are, 

 it seems to us, justified 

 in questioning this result, since it would place Gapitella in opposi- 

 tion to every other Annelidan type which has been studied. 



But Eisig records a feature from the post-trochophoral stage 

 which is not recorded of Polygordius, but which we have other 

 reasons for regarding as of an extremely primitive character, and this 

 is a ciliated groove stretching from mouth to anus. The same 

 feature recurs in Trochophore larvae belonging to quite different 

 groups of the animal kingdom. 



Fio. 116. — The free-swimming larva of Nereis limlata three 

 days old. A typical "Polytrochal" larva. (After 

 Wilson.) 



an, anus ; cA, chaetae ; ciZ, additional ciliated rings ; gl, gland 

 cells ; jjar, parapodia ; t.c, tentacular arms. 



OLIGOCHAETA 



In the Oligochaeta the prototroch is never developed at all : the 

 embryo develops into a blastula, which is converted into a gastrula by 

 invagination or by epibole. The mesoderm arises as a pole cell from 

 the lip of the blastopore which divides into two, and these two halves 

 proceed immediately to give rise to the mesodermic bands. The 



