138 LIBBIE H. HYMAN 



riorly, presence of a pair of transverse blood vessels in each 

 segment, and, in some cases, reversal of the direction of blood 

 flow, such as occurs in the biaxial tails. Tail characteristics 

 predominate but the cephaluran structure differs from a tail in 

 the absence of the anus, and the presence of the nerve mass. 

 Oephaluran outgrowths arise in pieces from posterior levels only. 



6. Biaxial tails. The regeneration of tails at both ends of 

 pieces has long been known in Lumbriculus; like the preceding 

 type of structure, biaxial tails arise in pieces from posterior regions 

 only. Both tails are identical in structure, the anterior tail 

 being however usually shorter, (fig. 20 g) ; and the blood always 

 flows from the tip of each towards the middle of the piece. 



7. Multiple outgroivths. In this category I have grouped all 

 those cases in which multiple structures arise at the cut surface. 

 Such multiplication of parts is limited to anterior regeneration. 

 The commonest case of this kind is the duplication of the head ; 

 and all gradations are found from two completely separate heads 

 to cases where only the prostomia are separate, as in figure 21. 

 The double structure may consist, however, not of two heads, 

 but of head and a tail (fig. 22) ; this is not a case of axial hetero- 

 morphosis, for here the tail grows out in correlation with the 

 new head, while the biaxial tail develops in correlation with the 

 old piece. If both parts of a double outgrowth develop with 

 equal rates of metabolism, then neither one will be able to con- 

 trol the other, and both will give rise to heads ; but if one of the 

 outgrowths attains higher metabolic activity than the other, it 

 will become a dominant part, a head, compelling the other out- 

 growth to develop as a subordinate part, a tail. The result is a 

 head with two tails. Pieces of Lumbriculus may give rise to 

 three or four such outgrowths at their anterior ends, some of 

 which are heads of various kinds, and some of which are tails. 



To illustrate now the distribution of these different types of 

 anterior outgrowth along the axis, the results of a series of twenty- 

 five worms cut up into sixteenths are given in table 1. As the 

 first sixteenth pieces include the original head, and the last 

 sixteenth pieces all died, they are omitted from tlie table. 



