PROCESS OF REGENERATION 135 



the anal segment is regenerated, although such pieces are never 

 tailless. With the exception, then, that extreme anterior pieces 

 may remain tailless, all levels of the Lumbriculus body regener- 

 ate normal posterior ends, but the amount regenerated decreases 

 along the axis. 



In anterior regeneration, the character of the anterior out- 

 growth depends upon the level of section, and shows all grad- 

 ations from a normal head to a normal tail. In an axial series 

 of short pieces, the most anterior ones regenerate normal heads, 

 usually of seven segments, but not infrequently hypo- or hyper- 

 meric. Pieces farther back tend to produce structures which 

 depart more and more from the normal head; and pieces from 

 posterior regions give rise to various types of abnormal, or, as I 

 prefer to call them, inhibited structures, among which are 

 represented all gradations from a normal head to a normal tail. 

 I have attempted to classify these various kinds of anterior struc- 

 tures into several categories, although it must be understood 

 that they grade into each other perfectly. The following de- 

 scription together with the accompanying figures will give some 

 idea of their appearance. 



1. Norinal heads. This head has a large well-developed 

 prostomium, mouth, pharynx, supraoesophageal ganglia with 

 circumoesophageal connectives, and usually consists of seven 

 segments, of which six are setigerous (fig. 20 b). It resembles 

 in all respects the head of the animal as it occurs in nature (fig. 

 20 a), except that the transverse pigment stripes are lacking on 

 the regenerated head. The typical number of segments in the 

 normal head is seven, but in short pieces, the number of regener- 

 ated segments varies from four to nine. Normal heads are 

 regenerated at any level of the Lumbriculus body, but in short 

 pieces, the frequency of their occurrence decreases, and the 

 tendency to hypomerism increases along the antero-posterior 

 axis. 



2. Hijpoprostomic heads. These differ from the normal in 

 that the prostomium is reduced in size, and frequently abnormal 

 in shape (fig. 20 c). The hypoprostomic head has a brain (which 

 has not been investigated histologically) mouth, and pharynx, 



