215 



differ somewhat from those of the Dicyemidae, yet on the whole they 

 develop like them. The infusoriform embryos of Conocyema resemble 

 those of the Dicyemidae. 



General Coxsiderations. 

 There are so many common features in the structure and 

 development of the Orthonectidae and Dicjemidae that we 

 cannot doubt the relationship of the two groups. Their 

 relations to the other divisions of the animal kingdom, on 

 the contrary, are more difficult to understand. In view of 

 th6 fact that they are said to be composed of only two germ- 

 layers, an attempt was made to create out of them a new 

 division of the animal kingdom, that of the Mesozoa, which 

 would be interpolated between the Protozoa and the Metazoa 

 (E. VAN Beneden, Julin). Since it is only parasitic forms 

 with which we have to do, such an explanation seems to us 

 venturesome at least, and we consider it more probable that 

 these simply constructed animals are Platyhelminthes which 

 have become degenerated through parasitism (Leuckart, 

 Metschnikoff, Whitman). 



The resemblance of the female of the Orthonectidae to the embryos of 

 the Distomidce has already been pointed out. The theory that such 

 embryos have reached sexual maturity has nothing improbable about it, 

 for such cases are also known elsewhere in the animal kingdom. Thus 

 Dinophihis is evidently to be regarded as an annelid larva which has 

 become sexually mature, and it is appropriate for comparison here, 

 inasmuch as its males have degenerated to nearly the condition of the 

 Orthonectidae and Dicyemidae (comp. infra, p. 313). The intestine and 

 other features of a higher organization having been lost, they present 

 within the body only a large testicular sac, similar to the males of the 

 Orthonectidae, which, to be sure, remain at a somewhat lower stage. 



If we regard the Orthonectidae and Dicyemidae as degenerated forms, 

 then the Orthonectidae, with their central cell-mass, would represent the 

 higher grade, whereas the Dicyemidae, in which only one central cell 

 is present, are more degenerate. However, in these also the inner por- 

 tion becomes multicellular as soon as the formation of the germ cells by 

 the division of the axial cell begins. 



Literature. 



Orthonectid^. 

 1. Braun, M. Die Orthonectiden. Centralbl. Bakt. u. Parasiteiikunde. 



Bd. ii. 1887. 



