PROCESS OF REGENERATION 133 



meric heads, consisting of only four segments, of which three 

 are setigerous, but normal in all other respects, occur occasion- 

 ally; but heads containing less than four segments have never 

 been observed. Hypermeric heads are I'are, but I have observed 

 a few, consisting of six, and, in one case, seven segments; in 

 such heads, the setae tend to be irregularly disposed, so that 

 one is uncertain of the boundaries of the segments. The hypo- 

 meric and hypermeric heads do not regulate to normal, at least 

 in the time which I observed them, nor is the abnormal number 

 of head segments inherited in asexual reproduction. In poste- 

 rior regeneration, there arise, not infrequently, abnormalities in 

 the shape of the gill pavilion, and in the size, and number of the 

 gills — one, two, or three appearing instead of the normal four; 

 but in the case of posterior structures, .regulation to normal 

 usually occurs. 



In regard to the regeneration of Dero, then, the important fact 

 is that complete anterior regeneration occurs ever^^where along 

 the axis (with the exception of the extreme posterior end) . 



4- Regeneration in Lwrnhrimilus inconstans 



Lumbriculus inconstans is undoubtedly the most favorable 

 oligochaete for experimental work on account of its large size, 

 high capacity for regeneration, low mortality, and, most impor- 

 tant of all, because it exhibits qualitative axial differences. 



It has this disadvantage, however, — that, owing to its peculiar 

 habitat, it can be collected only in the spring months; freshly 

 collected material is necessary for experimental work because 

 the vitality of the worms decreases when they are kept for any 

 length of time in the laboratory. Owing, therefore, to the rel- 

 atively short time during which material is available, my experi- 

 ments on this most instructive species are as yet incomplete in 

 many details. 



The genus Lumbriculus has long been noted for its high re- 

 generative capacity, and the species L. inconstans is no less 

 remarkable in this respect than are its relatives, L. variegatus, 

 a favorite object of research with European investigators, and 



