48 ' THE PROTOZOA 



becoming smaller and smaller as more and more water is expelled. 

 The nucleus appears unaltered, except for a very slight reduction in 

 size (Fig. 17). In this condition the animal can withstand a long 

 period of desiccation, or even extreme heat and cold, and, owing to its 

 minute size, it may be blown hither and thither until it reaches some 

 favorable spot where it may recommence active life. Water is then 

 absorbed, the cyst is ruptured, and the former active life begins anew. 

 Encystment may occur in some cases after a particularly heavy meal, 

 or more frequently, before reproduction by spore-formation or simple 

 division. 



I . Nutrition. 



The processes of nutrition, as in the higher animals, may be divided 

 into three stages: i, the capture and ingestion of food; 2, the 

 digestion of the ingested parts ; and 3, the ejection or defecation of 

 the undigested remains. 



Many of the Protozoa have no special apparatus for seizing and 

 ingesting food, but absorb it directly from the surrounding medium. 

 Thus many Mastigophora live, like the fungi, by absorption, through 

 the body walls, of fluids which hold in solution the products of decom- 

 position of other animal and plant forms. Others, as the Sporozoa 

 and some Ciliata, live like a tapeworm and other intestinal parasites, 

 upon the digested foods of the alimentary tract, or in nutritive fluids 

 in other cavities of different hosts. The Phytofiagellida, also, do not 

 ingest solid food, but, by the aid of chromatophores, they have the 

 power of manufacturing their food in the same manner as do the 

 green plants. The majority of Protozoa, however, take in solid food 

 through more or less definite regions of the body. In some of the 

 phytoflagellates there is a distinct mouth-opening, in addition to the 

 chromatophores, and such forms may combine both animal and plant 

 modes of nutrition. 



Food-taking has been carefully examined in connection with the 

 Ciliata, where many species living upon certain specific organisms 

 apparently select their food. Thus Maupas ('88) distinguishes forms 

 that are herbivorous, others that are carnivorous, and still others that 

 are omnivorous. The food that may be thus selected consists of all 

 sorts of lower plants, such as desmids, diatoms, zoospores, bacteria, 

 filamentous algae, etc., while among the animals the Mastigophora 

 and smaller ciliates are the most frequent victims, although rotifers 

 and small worms are often eagerly seized. Maupas believes that 

 the cause of these various adaptations in feeding should be sought 

 in the modifications of the mouth. " The mouth is, in short," he says, 

 " the dominating organ par excellence in the morphology and the 

 physiology of the Ciliata. Nutrition in its manifold phases in these 



