PROTOZOA. 9 



with fluid contents, which alternately increases and diminishes in 

 size, in a rhythmic manner. There may be more than one. 



More or less food, surrounded by fluid, is usually present in 

 the endoplasm, occupying spaces known as food-vacuoles. Gas- 

 containing vacuoles have also been observed in some specimens. 



PHYSIOLOGY. 



The functions carried on by Amoeba and, indeed, by all other 

 organisms may be conveniently considered under five headings. 



1. Nutrition, The food of Amoeba consists of small organisms 

 and organic particles, which its pseudopodia first come into con- 

 tact with, and then flow around. All parts of the body alike 

 serve for the reception of food. The complex chemical composi- 

 tion of the substances utilized, and their ingestion in the solid 

 state are worthy of notice, as typical animal characteristics. 

 Each mass enters the body surrounded by a small quantity of 

 water, and a food-vacuole is thus constituted, the temporary 

 breach in the protoplasm being at once closed up. Within the 

 body, mechanical and chemical influences are brought to bear 

 upon the food, it being subjected, on the one hand, to slow rota- 

 tion in the endoplasm, while on the other, either the protoplasm 

 itself or a digestive juice formed (secreted) by it reduces the 

 digestible portions to a state of solution, or fine division. The 

 food thus digested is then assimilated, that is, built up into living 

 protoplasm, while the indigestible and undigested remnants are 

 thrown out at any convenient point. 



Encystment. If conditions of temperature or food-supply are 

 unfavourable, Amoeba possesses the power of assuming a spherical 

 form, and then secreting a firm structureless coating or cyst, which 

 is possibly of a horny nature, and is a very considerable protec- 

 tion. 



2. Katafoolism. The complex protoplasmic molecules are con- 

 stantly breaking down into simpler substances, and potential 

 energy is thus transformed into kinetic. This process is, as a 

 rule, far more rapid in animals than in plants, and greater activity 

 is consequently displayed by the former. 



The products of Katabolism (Katastates) are : 

 a. Secretions, which are utilized in some way before passing 

 out of the body. It is probable, for example, that Amoeba 



