OF MICR O- OR GANISMS. 5 



the mode of multiplication: they are reproduced by 

 spores. In the animals of this group, the special 

 motor organs are wanting; these creatures therefore 

 generally move very little, or they present only move- 

 ments of which the principles are unknown. 



We shall successively describe the pseudopodia, 

 the vibratile cilia and the flagellum. 



The Pseudopod. The formation of pseudopodia 

 takes place chiefly in naked cells in cells lacking an 

 enveloping membrane, in the Sarcodines in general. 

 They can easily be studied in the Amoeba princeps, a 

 microscopic animal which is found in abundance in 

 fresh water containing organic matter in a state of 

 putrefaction. It has the aspect of a small gelatinous 

 mass, irregular, formed of a colorless substance, the 

 protoplasm. The chemical nature of protoplasm is 

 still very imperfectly understood; it is only known 

 that it is the result of a mixture of albuminoid mat- 

 ters, with an addition of water and mineral elements. 

 In the protoplasm of the amoeba exists a small rounded 

 and refracting mass, containing one or two bright cor- 

 puscles in its interior; this small mass is called the 

 nucleus, and the corpuscles the nucleoli. 



The form of the body of the amceba is rendered 

 very irregular by the fact that certain parts of the 

 mass lengthen, and form short and rounded protuber- 

 ances which are designated by the name of pseudo- 

 podia. It is by means of these pseudopodia that the 

 animal moves; it emits them in the direction in which 

 it is going, then it retracts them, while other parts of 

 the mass are in their turn elongated. The whole body 

 moves by creeping. The amceba in moving has the 

 aspect of a drop of oil moving along. To explain the 

 mechanism of this movement, it must be supposed 



