PROCESS OF REGENERATION 137 



ageal ganglia, and circumoesophageal commissures are absent, 

 and that the ventral nerve cord terminates in a mass lying be- 

 neath or in front of the blind end of the pharynx. It seems ver}- 

 probable to me that the brain and comissures do not develop 

 as distinct structures because the mouth is absent. The appro- 

 stomic head is always hypomeric, usually markedly so, and like 

 the preceding type is of most frequent occurrence in pieces from 

 posterior regions. 



4. Acephalic pieces. In these pieces, the cut end simply heals 

 over and no outgrowth takes place (fig. 20 e). These headless 

 pieces are rare in Lumbriculus, and are confined to posterior 

 levels of the body. 



In these four classes of structures, we have, as in Planaria 

 dorotocephala (Child 'lib) all gradations from the normal 

 head to the acephalic condition. There is a gradual reduction 

 of certain cephalic structures, — the prostomium, cephalic nerv- 

 ous system, mouth, and number of head segments, ending with 

 complete inhibition of head formation. The various types of 

 heads, are not, however, so definitely localized along the axis, 

 as is the case in Planaria, although in general, the percentage of 

 normal heads decreases along the axis, and the percentage of 

 inhibited heads increases. Lumbriculus further differs from 

 Planaria in that in place of acephalic pieces, posterior structures 

 may arise. I have distinguished two kinds of these. 



5. Cephaluran outgrowths. This interesting type of anterior 

 outgrowth appears to have, as the name adopted for it implies, 

 characteristics of both head and tail (fig. 20 f). In my opinion, 

 such an outgrowth starts as an inhibited head, an aprostomic 

 head, but is unable to maintain sufficient dominance over the 

 old piece to continue to develop as a head; its further develop- 

 ment is therefore that of a subordinate part, a tail. The ceph- 

 aluran outgrowths have a rounded or pointed end in which there 

 is a terminal nerve mass like that found in the aprostomic heads ; 

 like the latter, also, there is no opening to the exterior, neither 

 mouth nor anus. The tail characteristics of these structures are 

 the following: large number of segments, for they are nearly 

 always hypermeric, decrease in the size of the segments ante- 



