650 HAROLD HEATH, 



the prototroch and separates the head segment from that of the 

 trunk" 1). 



Finally regarding the relation of the head and trunk it follows 

 from the above discussion that the Annelid pro- and peri-stomium 

 are modified from the head vesicle and a small portion of the trunk. 

 The prostomium probably arises from the head vesicle alone and is 

 formed therefore from first quartette cells. The peristomium however 

 is derived partly from the cells of the head vesicle and partly from 

 the trunk. According to diagram E, e it includes all the various 

 quartettes in the trunk save the first somatoblast which with the ex- 

 ception of a small portion lies posterior to it. 



The "head" therefore is phylogenetically the older 

 portion of the Annelid while the greater portion of the 

 trunk is comparatively a late formation. Metameric 

 segmentation belongs to the trunk region and is there- 

 fore secondary and I should look upon the pro- and 

 peri-stomium as one segment or better perhaps as the 

 portion ofthetrochophorewhich has undergone no seg- 

 mentation. 



Stanford University, Palo Alto, 

 California, May 1898. 



1) Dr. Mead has kindly sent me sections of the Aniphitrite trocho- 

 phore which show from one to three trunk segments clearly indicated 

 in the ectoderm while the mesoblast remains entirely or but little dif- 

 ferentiated. And he also writes me that Clymenella exhibits essentially 

 the same characters as Amphitrite. This I also find true for a three- 

 segmented Nereis (?) larva taken in abundance in the plankton of 

 Monterey Bay. 



