VII 



ANNELIDA 



155 



In the two following points, namely, the primitive situation of the 



mother cells of the meso- 



derm in the wall of the 



gut, and the persistence 



of the protonephridia, 



Eupomatus can fairly 



claim to show, on the 



whole, a more primitive /^A 



development than even 



Polygordius. A detailed 



study of the develop- 

 ment of this form, or of 



that of the allied genus 



Pomatoceros, is greatly 



to be desired. 



Turning now to 



Nereis as an example of 



the " telescoped " form 



of development, de- 

 scribed in detail by 



Wilson (1892), the first 



difference that strikes 



us as compared with 



Polygordius is that the 



macromere D is, from 



the first, distinguishable 



from the other three 



by its greater size. The first quartette of micromeres are, however, 



equal in size and their development is, 

 in broad terms, the same as in Poly- 

 gordius. In' each quadrant, however, 

 one of the trochoblasts does not develop 

 cilia, but joins the intermediate girdle 

 cells to form part of the covering of the 

 upper hemisphere. 



When, however, the second quartette 

 of micromeres are given off, the pos- 

 terior one, 2d, called by Wilson " the 

 first somatoblast," is very much larger 

 than its sisters, and after a few pre- 

 liminary divisions proceeds to bud off 

 that ventral plate of ectoderm cells 

 which covers the post-trochal " body " 

 of the worm (Fig. 115). 



The third quartette is normal, but 

 in the case of the fourth quartette, 



4d, termed by Wilson the second somatoblast, is formed some 



considerable time before its sisters, and at once divides into right 



PN 



FIG. 114. Diagrammatic saggital section of fully-grown 

 Trochophore larva of Eupomatus to show the relative 

 position of the protonephridia and the coelomic 

 rudiment. (After Shearer.) 



Letters as before, rn.es, larval mesoderm, consisting of ecto- 

 derm cells budded inwards from the second quartette. 



FIG. 115. Stage in the segmentation 

 of the egg of Nereis limbata viewed 

 from above, showing a laeotropic 

 spiral cleavage of the egg. (After 

 Wilson.) 



j>, polar bodies. 



