PROCESS OF REGENERATION 125 



pharynx, which is completely lacking in chloragogue cells. The 

 outer wall of the remainder of the alimentary tract is composed 

 of these chloragogue cells, which contain numerous brown drop- 

 lets, apparently metabolic products (since they increase with 

 age), which give to the intestine a characteristic dark brown 

 color. There is thus a sharp line between the white pharynx 

 and the dark intestine, and this line is the boundary between 

 head and trunk. Other less obvious characteristics of the head, 

 which are mentioned by Beddard ('95, p. 11) in his discussion 

 of cephalization in oligochaetes, are: the absence of nephridia, 

 irregularity of the septa, absence of the circum-intestinal loops 

 of the circulatory system, and, in the naids, absence of the dor- 

 sal bundles of setae. 



That the head of the oligochaetes is a real imit is shown not 

 only by its morphology but by the fact that usually the head 

 segments only are regenerated, no matter how many anterior 

 segments are removed. This statement, although true in general, 

 requires modification, for the regeneration of the anterior end 

 varies according to the size of the piece and the region from which 

 it is taken. Thus Lumbriculus regenerates the typical number 

 of head segments on'y when the piece is of sufficient size. An 

 insufficient investigation of cases such as this has led some investi- 

 gators to question the existence of a definite head region in oli- 

 gochaetes. Thus von Wagner ('00) working on Lumbriculus 

 \'ariegatus says that the number of anterior segments regener- 

 ated in this form is not definite but varies from five to nine. 

 Semper ('76) seems to have been the first investigator to call 

 attention to the sharp differentiation of the head from the trunk 

 in oligochaetes; he observed it in the naids, in which forms it is 

 most obvious. Later Billow ('83) stated that although there is 

 indeed variation in the number of anterior segments regenerated 

 by Lumbriculus variegatus, yet one number — namely nine — pre- 

 dominates. Iwanoff ('03) supports Billow's conclusion in regard 

 to Lumbriculus — although he finds the number to be seven — 

 and has maintained positively that there exists in oligochaetes 

 a head region comprising a fixed number of segments. He uses 

 the same criterion for distinguishing the head as I do, namel}^, 

 the absence of chloragogue cells. 



