■ PROTOZOA 167 



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 lluid, the food vacuoles 

 (figs. 121, 150, etc., 11a), more rarely in the protoplasm itself. All in- 

 digestible portions are cast out after a time. This taking in and casting 

 out of foreign matter can take place in the naked IVotozoa at any point 

 of the surface, while in the more highly organized species when the outer 

 surface is hardened by a pellicle or a thin cuticula, there are definite open- 

 ings 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 rvto/)//an'wa;, 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 processes — pseudopodia, 

 ilagella, and cilia — there are Protozoa, like Stentor and the Vorticellidae 

 which have muscular fibrillce. The sensitiveness to light is often increased 

 by an eye spot, a small pigment body in wliich even a lens may occur. 

 More constant of cell organs are the contractile vacuoles (fig. 117, etc., cv), 

 rarely absent from fresh-water species, but commonly lacking from marine 

 forms. These have a definite place in the cell ; their number is approxi- 

 mately constant in most species; they exhibit extremely constant phe- 

 nomena. The walls contract and empty the fluid contents to the exterior, 

 often through a special duct. When one empties it completely disap- 

 pears and is formed again anew in a short time, and is filled with fluid 

 from the surrounding protoplasm. It thus resembles the contractile 

 vacuoles in the water vascular system (excretory organs) of the worms to 

 be described later. Apparently the contractile vacuoles are for the 

 elimination of injurious substances in solution produced by the vital pro- 

 cesses, among them possibly carbon dioxide, like a respiratory organ. 



The occurrence of such diverse differentiations, recalling organs and tissues, 

 gives such a complicated appearance and such a degree of specialization to the 

 protozoan body, that it was questioned for a time whether all could belong to a 

 single cell. Yet it was a mistake to doubt the unicellularity of the Protozoa, for 

 according to our conceptions of the cell, there is the capacity to develop in 

 many directions, to produce a kind of stomach, muscle fibres, sense apparatus, 

 skeleton and the like; although in the organization of the higher animals it 

 produces only a specific product (muscle cells, contractile substance, gland cells, 

 secretion). 



The \dtal phenomena of the Protozoa proceed from the protoplasm, 

 but mth a certain dependence upon the nucleus. If an infusorian or an 

 Anueba be cut into nucleate and anucleate portions, only the first can 



