A Contribut. to ttie Embryol., Life-liistoiy, and Classificat. of the Dicyeraids. 75 



In both cases these eells arise from the tvvo poles of the centrai or 

 axial cell. In the Dicyemids they lose their polar positiou and vvander 

 into the centrai celi ; in the Orthonectidae, they raaintain their originai 

 j)osition. Thefact that these polar cells becomeniuscle-fìbrils in one case, 

 and germ-cells in the other, is no objectiou to the opinion that they are 

 raorphological equivalents. It is on this ground that the Dicyemids 

 niaybesaidto bave a transient triploblastic stage, rep- 

 resented by au ectodermal layer, an axial endodermic 

 cell, and two intermediate mesodermic cells derived 

 frolli the two poles of the endodermi e celi. 



In the Orthonectidae there is a great reduction of the mesoderm 

 by atrophy , or otherwise, during the course of develupment; and 

 there can be little doubt that this reduction Stands in correlation with 

 an increased reproductive activity. In the Dicyemids we see an 

 abridgment of the sanie process in the fact that no permanent mesoderm 

 ever comes to development. 



Thus far no objection has been raised to the creation of a middle 

 division of the animai kingdom on the ground that ali the assumed 

 r epresentatives of this division are parasites. But I 

 think this must be admitted to be one of the unfortunate aspects of the 

 case , although we may not be able to point to undeniable evidence of 

 degeneration. But who will venture to assert that, before applying the 

 hypothesis of »degenerative evolution« , it is indispensable to find un- 

 mistakable marks of degeneration, such as are seen, for example, in the 

 development of Sacctdina^ Lertiaeocera^ Barnacles, etc.? That this 

 iiypothesis admits of a very wide application to the simpler forms of lite 

 has been made sufficiently clear by Dohrn (27) and Lankester (40). 

 When we find an animai in the form of a simple sack, filledwith repro- 

 ductive Clements , secured by position against enemies, supplied with 

 food in abundance, and combiuing parasitism with immobility, we bave 

 strong reasons for believing that the simplicity of its structure is more 

 or less the result of the luxurious conditions of life which it enjoys, even 

 if its development furnishes no positive evidence of degeneiation. 



As to the systematic position of the Dicyemids, I see no good rea- 

 son for doubting the general opinion that they are Plathelminths de- 

 graded by parasitism. Whether they and their allies, the Orthonectidae, 

 bave desceiided from ancestors represented now by such forms as Binoph- 

 ilus (Metschnikoff) , or from the Trematoda (Leuckakt), is a ques- 

 tion which further investigations must decide. 



