PROTOZOA. 183 



PHYLUM I PROTOZOA. 



All of the Protozoa are small ; some may be seen by'a sharp eye 

 as minute points, but the majority are so minute that they are 

 invisible except with a microscope. On the other hand, there are 

 a few which have a diameter to be measured by millimeters, this 

 being especially the case where hundreds of individuals are united 

 in colonies. 



This small size is a necessary result of the fact that the Protozoa 

 are single-celled animals. Like all cells they consist of that pe- 

 culiar substance, protoplasm, and they have the further cell attri- 

 bute, the possession of one or more nuclei. Being unicellular, it 

 follows that they lack true tissues and true organs. They lack 

 alimentary canal, nervous system, sexual organs, etc. The funda- 

 mental functions of nourishment, sensation, movement, and repro- 

 duction are performed more or less directly by the protoplasm. 



In nutrition, in so far as it is not produced by substances in 

 solution, foreign particles pass into the protoplasm and are digested 

 by it. They usually lie during digestion in special collections of 

 fluid, the food vacuoles (figs. 120, 144, etc, no), more rarely in the 

 protoplasm itself. All indigestible portions are cast out after a 

 time. This taking in and casting out of foreign matter can take 

 place in the lower Protozoa at any point of the surface, while in 

 the more highly organized species there are definite openings which 

 according to analogy with many-celled animals are spoken of as 

 mouth and anus, or more precisely, cytostome and cytopyge. The 

 mouth may connect with a tube, the oesophagus or cytopharynx, 

 which ends free in the protoplasm. 



Structures may occur within the protozoan cell which recall 

 the organs of higher animals, and which are called cell organs. 

 While motion is usually produced by the protoplasm and its pro- 

 cesses -pseudopodia, flagella, and cilia there are Protozoa, like 

 Stentor and the Vorticellidae, which have true muscular fibrillae. 

 The sensitiveness to light is often increased by the formation of an 

 eye spot, a small pigment body in which even a lens may occur, 

 More constant of cell organs are the contractile vacuoles (fig. 116 ? 

 etc., cv), structures rarely absent from fresh-water species, but 

 commonly lacking from marine forms. These are distinguished 

 from the food vacuoles- by three characters: they have a definite 



