CHAPTER VIII. 



EOTIFERA. 



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 Piatyelminthes, while in other respects they possibly approach 

 the Arthropoda (e.g. Pedalion ?). The interesting Trochosphcera 

 ceqiiatorialis of Semper closely resembles a monotrochal polychastous 

 larva. 



Up to the present time our embryological knowledge is mainly 

 confined to a series of observations by Salensky on Brachionus 

 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 Colm 

 (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 Lacinularia developed without 

 previous fertilization. 



The following are the more important results of Salensky 's ob- 

 servations (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 development. 

 It will be convenient to treat separately the development of the 

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



