ANNELIDA 347 



is to be assigned to the head or to the trunk. In the 

 solution of this problem the condition of the mesoderm 

 plays a particularly important role. No real ctelom appears 

 to be formed in the portion lying in front of the mouth ; 

 on the contrary, the first pair of primitive segments is said 

 to surround the pharynx. If this were so, then a distinction 

 would really be established between the preoral and the 

 oral portions, and the latter would have the greater re- 

 semblance to the body segments. However, this distinction 

 is later obliterated owing to the fact that portions of the 

 mesoderm from the most anterior primitive segments migrate 

 into the preoral region and form its musculature. Thus 

 interpretations differ, inasmuch as the preoral part alone 

 (Kleinenbero) and also that together with the oral segment 

 (Hatschek) have been taken to be the head portion. More- 

 over, induced by the peculiar phenomena in non-sexual 

 reproduction, some authors have gone farther than this and 

 considered a greater number of segments (as many as six) 

 as the head portion of the worm (Semper, v. Kennel). The 

 first and last theories seem to us to go too far. Until the 

 final settlement of the question how the mouth and the 

 pharynx are related to the first primitive segment, we would 

 reckon the mouth region as belonging to the head of the 

 worm. 



The transition from the Trochophore-like stock-form to 

 the real Annelid ancestors (Archiannelida) took place by 

 considerable growth in length, whereby the trunk portion 

 of the worm became larger, and the primary head portion 

 less and less conspicuous. At the same time a change in 

 the mode of life took place, the pelagic being exchanged 

 for the creeping habit. 



The larval stages belonging to this transition are dis- 

 tinguished especially by the terminal growth of the body. 

 Near the posterior end of the body, which we can henceforth 

 distinguish as the anus-bearing terminal segment, is found 

 a zone of proliferation, from which new cell material is 

 continually being given off forward to the elongating trunk 

 portion. Since, at the same time with this growth in 

 length, the segmentation of the trunk is established, it 



