SOME PROBLEMS OF ANNELID MORPHOLOGY. 7 1 



tion of bilateral animals from the radiate (coelenterate) 

 forms, which by common consent are considered to have 

 been their progenitors. 



At present, apparently, the data do not exist for 

 a trustworthy decision between these two conflicting 

 views ; though, as a matter of fact, most practical em- 

 bryologists adopt one or the other as a working hypoth- 

 esis. Speaking for myself alone, and judging from the 

 development of annelids, the view that concrescence 

 is a wholly secondary process seems inadequate and 

 opposed to many important facts. There are forms — 

 the earthworm, for instance — in which there is little 

 food-yolk, and yet a nearly typical concrescence takes 

 place. Furthermore, a nearly complete series may be 

 traced, from such typical cases of complete concrescence 

 as Clcpsine or R/iyjicheimis, to the opposite extreme of 

 PolygGrdiiis, in which there are no "germ-bands" and 

 no concrescence save that of the mesoblastic bands. It 

 is precisely these bands, however, that form the most 

 important element of the germ-bands in C/epsi7ze, etc., 

 inasmuch as the development of the other parts is, as I 

 have said, moulded upon them. There is no logical jus- 

 tification for making any fundamental distinction be- 

 tween complete concrescence (i.e. of the germ-bands), 

 and partial concrescence {i.e. of the mesoblastic bands). 

 The latter process is, however, one of the most charac- 

 teristic features in the development of all annelids, 

 whether possessing food-yolk or not ; and this, in my 

 opinion, is fatal to the theory in question. 



Let us turn, therefore, to the second view. If it be 

 true that the origin of concrescence goes back to the 

 origin of bilaterality, then our inquiry must be extended 



