THE EYES OF ROTIFERS 89 



uppermost, having an apical plate bearing a tuft of vertical cilia. This 

 plate has a pair of short tentacles and eye-spots which lie over the nerve- 

 centre or ganglion. There are, moreover, usually two circlets of vibratile 

 cilia which surround the body, one lying above or in front of the mouth — 

 preoral — the other below or behind the mouth — postoral. The mouth is 

 placed on one side of the body, namely the ventral aspect, and the anus 

 is at the lower pole or posterior extremity. A pair of tubes, the excretory 

 organs or nephridia, are present and in the female adult an ovarium is 

 connected by an oviduct with the cloaca. 



Taking the adult form of Brachionus rubens as an example, we find 



cu 

 - / d - e P ,nph. 



mth - c" 



ov ' 



c ph. m st ovy vt. ovd. cv 



Fu 



57. — Diagram of a Rotifer, showing the Relative Positions of the 

 Brain, Median Eye, and other Viscera. 



(Redrawn from Parker and Haswell.) 



that in spite of its minute size there is a high degree of differentiation. 

 As regards the nervous system, a large cerebral ganglion is present at the 

 anterior end of the body which lies above or dorsal to the alimentary 

 canal. Where the cerebral ganglion comes into contact with the body- 

 wall, there is a median eye-spot of small size and red. Three other organs 

 considered to be sensory are known as tactile-rods ; one of these, bearing 

 stiff hair-like organs, projects from the dorsal surface just behind the 

 " wheel " or " trochal disc " ; the other two are paired and situated on 



