10 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



secretes something which acts chemically upon the food, 



and corresponds to the digestive juices of other forms. 

 I. Excretions (waste-products), which pass out of the body 



at once. These are water, carbon dioxide (C0 2 ), and 



compounds of ammonia. 



Katabolism is, practically, a process of oxidation, the result 

 being that animals, like most plants, require a supply of free 

 oxygen, and the term Kespiration, or breathing, is applied to the 

 taking in of oxygen with concomitant excretion of carbon dioxide 

 (and water). 



The contractile vacuole, which is alternately enlarged to a certain 

 maximum size by the gradual aggregation of liquid, and obliter- 

 ated by the contraction of the surrounding protoplasm, appears 

 to communicate with the exterior. It probably serves as a recep- 

 tacle in which can collect, at one time, oxygenated water from 

 the exterior, at another time waste-products. The general surface 

 of the body is also doubtless respiratory, oxygen diffusing in and 

 carbon dioxide diffusing out. 



3. Reproduction (Fig. 1, A-C). This is asexual, and usually 

 effected by binary fission, the animal splitting into two equal parts. 

 The nucleus elongates, becomes dumb-bell-shaped, and separates 

 into two, each half being surrounded by a moiety of the proto- 

 plasm, which has meanwhile been gradually constricted. The 

 two new individuals grow to the adult size, divide again, and so 

 on, there being no known limit to the process. 



An Amoeba, in fact, instead of dying, divides into two new and 

 vigorous individuals. Hence it has been said to be, in a sense, 

 immortal. 



4. Contractility is one of the primary properties of protoplasm, 

 in virtue of which a change of form is effected. The process does 

 not involve diminution in volume, since reduced breadth in one 

 or more directions is made up for by equivalent increase in other 

 directions. In Amoeba there are irregular contractions of the 

 protoplasm leading to the formation of pseudopodia. Where one 

 of these is about to be thrust out, the exoplasm (which is probably 

 specially contractile) is protruded as a clear knob, into which, as 

 it increases in size, the endoplasm enters. An anterior progressing 

 region, actively flowing into pseudopodia, and a more passive 

 following region, can be made out in some Amoebae. These regions 

 are in ordinary cases determined by the direction of movement, 



