554 R. C. PUNNBTT. 



appendage joined it. He gives no account of the strnctuve 

 of the organ, though one would suppose that it contained no 

 portion of the alimentary canal. 



In Micrella, on tiie other hand, the appendage joins the 

 body ventral to the anus, and contains neither gonidial 

 pouches, intestine, nor outer longitudinal muscle layer, whilst 

 the vascular system in it is rudimentary. In the liglitof such 

 conflicting evidence it can only be conjectured that we are 

 probably not dealing with homologous structures in each case, 

 but that the caudal appendage in Heteronemerteans may have 

 an entirely different morphological significance — unless, indeed, 

 the anus is not homologous in the diiferent members of the 

 group. Owing to the fact that the primary subdivisions of 

 the great family oF the Lineidas are based upon the presence 

 or absence of a caudal appendage, the study of its structure 

 in a number of forms would be of the highest importance 

 for the systematist, whilst at the same time it might be ex- 

 pected to throw some light upon the morphological signifi- 

 cance of a very puzzling and enigmatical formation. 



One of the most interesting features connected with 

 Micrella is the light which it throws upon the relations of 

 the two Heteronemertean families — the Eupoliidte and the 

 Lineidte. In his monograph (p. 715) Burger has sketched a 

 family tree of the group. From it may be seen that he 

 dei'ives the Lineidas directly from a form such as Eupolia, 

 In the last-named genus we find an excretory system with 

 many ducts, such as occurs in many Lineidse. Micrella 

 alone in this family presents a condition of this system ap- 

 proaching that of the Protonemerteans, from which all the 

 Heteronemerteans are probably to be derived. But Micrella 

 already possesses the characteristic head slits, consequently 

 we must suppose that the Lineida3 branched off the common 

 stock before the type of excretory system with many ducts had 

 been evolved, and that this latter type has arisen independently 

 in the two families. The family tree given by Burger must 

 therefore be amended somewhat in the way which the accom- 

 panying scheme indicates, 



