126 LIBBIE H. HYMAN 



My own observations on oligochaetes have convinced me that 

 in this group a definite head region is present. It is most sharply 

 differentiated in the naids, where it generally consists of five 

 segments (Dero, Stylaria, Nais). The statement of some 

 authors (Iwanoff, '03) that these forms have but four head 

 segments is probably due to a failure to count the first segment, 

 which, as in all oligochaetes, bears no setae. In Tubifiex tubifex, 

 there are five head segments; in another species of Tubifex 

 which lives in the temporary ponds along with Lumbriculus 

 (probably Tubifex multisetosus) there are ten head segments. 

 Limnodrilus has four, and Lumbriculus inconstans seven head 

 segments, although in the latter they are not very sharply de- 

 fined because the heavy pigmentation of the anterior part of the 

 body conceals the division between head and trunk. In the 

 Lumbricidae, also, there appears to be a definite number of 

 cephalized segments; thus in AUolobophora foetida according to 

 the account of Morgan ('97) ,the number is four or five, and in A. 

 terrestris (Hescheler, '98), four. I have been able to find but 

 one exception to this general rule; this is the case of Criodrilus, 

 in which form there is apparently no definite cephalization since, 

 according to Tirala ('12), it regenerates up to 28 segments ante- 

 riorly, replacing, to a considerable extent, the number removed. 

 However, from the data presented by Tirala, it appears that even 

 in Criodrilus there is a tendency to regenerate a certain number 

 of segments, namely, about fifteen, and that with more detailed 

 work on a larger number of individuals this would appear more 

 clearly. I am therefore convinced from any own observations 

 and those of others that the differentiation of a certain number 

 of anterior segments as head segments is practically universal 

 among oligochaetes. 



2. General considerations on regeneration in oligochaetes 



Regeneration in oligochaetes is always by outgrowth; the 

 head is never replaced within the old tissue, as often occurs in 

 Protozoa, Coelenterates, and flatworms. The process of re- 

 generation, as it occurs in Dero limosa, for example, is as follows. 



