ANNELIDA 



349 



become possible, since the longitudinal stems in the two 

 cases could be looked upon as corresponding to each other 

 (whereby we even have in mind a former connection of the 

 permanent nephridia with the head kidney). At all events, 

 the anatomy and development of the Annelid body permit 

 the establishment of the interpretation of the entire body 

 as an individual. Just as in the consideration of the tape- 

 worm chain we were induced by the comparison with un- 

 segmented forms to refer the entire chain to an unsegmented 

 individual,^ and, on the other hand, to see in the proglottis, 

 not a complete individual, but only the abstricted hinder 

 portion of the body of the Cestode, in the same manner, 

 and with much more reason, we adhere to the individuality 

 of the Annelid body. We can accordingly recognize in 

 metameric segmentation only the regular repetition of certain 

 groups of organs in the trunk at uniform intervals. 



In the question of the origin of the metameric segmenta- 

 tion we shall have to ascertain whether the synchronism 

 of the terminal growth of the body and the appearance of 

 metameric segmentation correspond to a palingenetic con- 

 dition. In other words, in the hypothetical ancestral form 

 were new segments successively added behind during in- 

 crease in length, so that forms with many segments arose 

 from those with few by gradual increase in the number of 

 segments ? The fact that the growth of the body in length 

 by the formation of new segments at the posterior end is 

 typical in all Annelids and the forms derived from them 

 (Arthropoda) is an argument in support of this theory. In 

 that case we might perhaps be inclined to the opinion, as 

 stated by Hatschek, that in ancestral forms enlarging by 

 terminal growth the diiferentiation, originally progressing 

 continuously, became intermittent, and thus reached the type 

 of the metameric animal. But another view may also be 

 maintained, and, as it seems to us, with quite as much 

 justice — a view which is based upon the assumption that 

 at first an unsegmented, elongated ancestral form was pro- 

 duced by terminal growth, whereupon the entire body be- 



1 There is a considerable difference between this and the process of 

 strobilization. 



