vi ANIMALS AND PLANTS 193 



But Paramcecium is so huge a creature compared 

 with those hitherto discussed it reaches -j-^ of 

 an inch or more in length that there is no diffi- 

 culty in making out its organisation in detail; 

 and in proving that it is not only an animal, but 

 that it is an animal which possesses a somewhat 

 complicated organisation. For example, the sur- 

 face layer of its body is different in structure from 

 the deeper parts. There are two contractile 

 vacuoles, from each of which radiates a system of 

 vessel-like canals ; and not only is there a conical 

 depression continuous with a tube, which serve as 

 mouth and gullet, but the food ingested takes a 

 definite course, and refuse is rejected from a 

 definite region. Nothing is easier than to feed 

 these animals, and to watch the particles of indigo 

 or carmine accumulate at the lower end of the 

 gullet. From this they gradually project, sur- 

 rounded by a ball of water, which at length passes 

 with a jerk, oddly simulating a gulp, into the 

 pulpy central substance of the body, there to cir- 

 culate up one side and down the other, until its 

 contents are digested and assimilated. Neverthe- 

 less, this complex animal multiplies by division, as 

 the monad does, and, like the monad, undergoes 

 conjugation. It stands in the same relation to 

 Heteromita on the animal side, as Colecchcete does 

 on the plant side. Start from either, and such an 

 insensible series of gradations leads to the monad 

 that it is impossible to say at any stage of the 



199 



