CHAPTER VIII. 

 ROTIFERA. 



FOR many reasons a complete knowledge of the ontogeny of 

 the Rotifera is desirable. They constitute a group which retain 

 in the trochal disc an organ common to the embryos of many 

 other groups, but which in most other instances is lost in the 

 adult state. In the character of the excretory organs they 

 exhibit affinities with the Platyelminthes, while in other respects 

 they possibly approach the Arthropoda (e.g. Pedalion ?). The 

 interesting TrocJiosplicera ceqiiatorialis of Semper closely re- 

 sembles a monotrochal polychaetous larva. 



Up to the present time our embryological knowledge is 

 mainly confined to a series of observations by Salensky on 

 BmcJdonus urceolaris, and to scattered statements on other larval 

 forms by Huxley, etc. 



In many cases Rotifers lay summer and winter eggs of a 

 different character. The former are always provided with a thin 

 membrane, and frequently undergo development within the 

 oviduct. They are hatched in the autumn. The winter eggs 

 are always provided with a thick shell. 



The summer eggs are of two kinds, viz. smaller eggs which 

 become males, and larger, females. On the authority of Cohn 

 (No. 232) they are believed to develope parthenogenetically. 

 Males are not found in summer, and only seem to be produced 

 from the summer eggs. Cohn's observations, especially on 

 Conochilus volvox, are however not quite satisfactory. Huxley 

 (No. 234) came to the conclusion that the winter eggs of Lacinu- 

 laria developed without previous fertilization. 



The following are the more important results of Salensky's 

 observations (No. 236) on Brachionus urceolaris. 



The ovum is attached by a short stalk to the hind end of the 

 body of the female, in which position it undergoes its develop- 

 ment. It will be convenient to treat separately the development 

 of the female and male, and to commence with the former. The 



