chap, iv.] ROTIFERS. 119 



laterally, and are caused, by the contraction of the 

 muscles connected with them, to work upon two 

 centrally set pieces, which may be regarded as forming 

 an anvil. Notwithstanding the minuteness of these 

 forms, it has been possible to form some idea as to 

 the character of the secretions of their digestive cells ; 

 red monads swallowed by them exhibiting a bright red 

 colour in the stomach, thanks, apparently, to the acid 

 reaction of the gastric juice of these forms ; in the 

 other parts of the intestine they have been seen to be 

 of a dark or brown-red colour, owing to the neutral or 

 alkaline reactions of the contents of that region 

 (Colin). 



The characters of the digestive tract of the 

 Rotifers present us with several instructive phe- 

 nomena, for we find that in the males, which are 

 always smaller than the females, the intestine is 

 nothing more than a solid cord of cells, while some- 



O ' 



times there is in the females themselves an indication 

 of degradation in the arrested development of the 

 terminal portion of the gut, and the consequent return 

 to the lower aproctous condition or stage in which 

 an anus is wanting. Nor is this all ; while the males 

 of Nematoid worms are distinguished from the females 

 by having the generative ducts opening to the exterior 

 by a passage common to them and the intestine, the 

 Rotifer among Vermes presents an arrangement which 

 is exceedingly common among the Vertebrata ; that is, 

 the possession of an enlargement or cloacal chamber 

 into which there open not only the digestive and 

 generative tubes, but also the canals of the excretory 

 system. 



The fixed Bryozoa likewise obtain their food by 

 means of the currents of water which they set in 

 motion with the cilia that cover the surfaces of their 

 protrusible tentacles ; in a number the mouth is 

 guarded by an outgrowth (epistoine) which has a 



