492 CHARACTERS OF INVERTEBRATE ANIMALS 



It therefore appears that all the functions of life can be, and 

 are, performed by a single cell, this having to discharge the duties 

 which in higher organisms are distributed between different com- 

 plex organs and tissues. 



It is convenient to divide the Protozoa into three groups: — 

 I. Infusorians (Infusoria); 2. Amoeba-like Protozoa (Rhizopoda); 

 and 3. Gregarines (Sporozoa). 



Group I. — Infusorians (Infusoria) 



If boiling water is added to chopped hay or other vegetable 

 matter, and the infusion so procured allowed to stand for some 

 time, it will begin to putrefy, and a large number of minute active 

 creatures will make their appearance in it. The same thing would 

 happen if the infusion were of animal nature. The term Infusoria 

 was first applied to the minute forms noted in various decomposing 

 substances of the kind just mentioned, but they are by no means 

 invariably associated with putridity, and abound both in salt and 

 fresh water, or even on damp soil and vegetation. 



A very common form is the Slipper Animalcule {Paramcecium) 

 (fig. 301), which is readily obtained by making an infusion of hay 

 as described. It is an active whitish creature, just visible to the 

 naked eye. Placed under the microscope, it will be found to 

 possess an elongated body, which, unlike Amoeba, has a definite 

 shape owing to the fact that the outer layer of the protoplasm 

 (ectosarc) is firmer than the rest (endosarc), and covered by a thin 

 elastic membrane or cuticle. As another consequence of this, food 

 cannot be taken into, or solid waste ejected from, any part of the 

 body, but there is on one side a depression which leads to a mouth, 

 and this again into a very short gullet, that ends abruptly in the 

 soft internal protoplasm (endosarc). Not far from this depression 

 there is a spot where the cuticle is absent, and the solid refuse 

 from the food can pass out to the exterior. 



The presence of a cuticle, and the firm nature of the outer 

 protoplasm, prevent the formation of the blunt lobes (pseudopods), 

 which, as we have seen, serve in Amoeba as organs for obtaining 

 food and effecting locomotion. Their place is here taken by a 

 uniform covering of cilia, which are protruded through minute 

 holes in the cuticle. By them the animalcule is rowed about from 

 place to place, and they also set up currents by which food particles 



