xni ROTIFERA 427 



AFFINITIES OF THE ROTIFERA 



Zelinka endeavours to show that the course of development which 

 he describes proves, that Callidina, and therefore presumably all other 

 Rotifera, show affinities to the Trochophore larva of Annelida and 

 Mollusca ; that, in a sense, they are what Huxley termed persistent 

 Trochophore larvae. That there is a general resemblance between 

 certain Rotifers such as Callidina when adult, and Trochophore 

 larvae cannot be denied ; but if this resemblance be an indication of 

 real affinity, then we must regard the early development of Rotifera 

 as profoundly modified ; for there is no resemblance between the 

 early development of this group and the early development of a 

 typical Trochophore larva, with its specialized cleavage, characterized 

 by the formation of four macronieres and three quartettes of ecto- 

 derrnic micromeres given off from near the animal pole of the egg. 

 The small cells in the Rotiferan egg are given off, one by one, from 

 the -vegetative pole, and rotate round the posterior end of the egg on 

 to the dorsal surface. The excretory organs of the Rotifera look like 

 the archnephridia of the Trochophore larva, but their origin from the 

 ectoderm has yet to be demonstrated. Unlike all other nephridia 

 they do not open to the exterior, but discharge into an endodermic 

 sac the bladder, which we may compare to a coelomic sac, which 

 still retains its opening into the gut. The early differentiation of the 

 rudiments of the genital organs is a common feature in animals 

 without larval development and with a very short life-cycle. On the 

 whole, we may conclude that much more work remains to be done on 

 the development of types of Rotifera with different forms of trochal 

 disc, before the hypothesis of the affinity of the group with Trocho- 

 phore larvae can be regarded as substantiated. 



LITERATURE REFERRED TO 



Jennings. The Early Development of As^lanchna Herricki. Bull. Mus. Coinp. 

 Zool. Harvard, 1895. 



Zelinka. Studien Uber Radertiere III. Zeitschr. fur wiss. Zool. vol. 53, 1892. 



