INVEUTEKRATA. 401 



the larger structures which we shall meet in Hydra (chap, 

 xxiv., 4, nematocysts), and like them are means of attack 

 on or defence from other small creatures, though differing 

 from them in that a multitude are developed from a portion 

 of the single cell, instead of each one in a separate cell. 



3. Cell-mouth and Cell-anus. At the bottom of the 

 ventral groove there is a small area where cuticle and 

 cortex are both wanting. This is the only point at which 

 ingestion of food-particles can take place, and it may there- 

 fore be called the cell-mouth. It is obviously quite a different 

 thing morphologically from the mouth of a multicellular 

 animal; its function, however, is the same. The cilia in 

 the groove sweep all solid particles along with a current of 

 water towards this point, and here the former are taken 

 into the medullary protoplasm along with a drop of water, 

 as a food-vacuole. The food-vacuole travels along in the 

 medulla and undergoes a kind of circulation through it, while 

 the food-particle is being digested. Eventually all that re- 

 mains un absorbed is cast out at a point near the cell-mouth, 

 called the cell-anus or from the fact that the medulla is 

 only exposed here at the moment of defsecation and not 

 permanently as at the cell-mouth the potential cell-anus. 



4. Contractile Vacuoles. Paramecium has two con- 

 tractile vacuoles one anterior and one posterior. These 

 show a more complex structure than that of Amoeba, for 

 while the vacuole is refilling, a number of radiating tubes 

 can be seen along which the water is plainly being conveyed 

 into the vacuole. Otherwise the action of these is exactly like 

 that in Amoeba, and there is no doubt that they serve the 

 same function, viz. the elimination of water containing in 

 solution some of the waste-products of katabolism. 



5. The Two Nuclei. There are two nuclei, embedded 

 in the medullary protoplasm ; but these two nuclei have a 

 very different significance from the two or more sometimes 

 found in an Amoeba. They differ in size and appearance, 

 and also, as we shall see presently, in function. It is con- 

 venient to distinguish the larger as the meganucleus, and 

 the smaller as the micronucleus. 



ZOOL. 26 



