THE STUDY OF ZOOLOGY 11 



etc. Irritability refers to this ability to respond to stimuli of 

 various sorts. Knowledge of the external world is gained from 

 influences which stimulate part or all of the sensory mechanism 

 of an animal. Movements are the result of stimulation of the 

 motor apparatus by nervous impulses acting on muscle tissue. 

 The flow of digestive juice may be invoked by the chemical 

 stimulation of the walls of the digestive canal by food substances. 

 The entire mechanism for the coordination of functional activ- 

 ities in different parts of the body depends ultimately on the 

 irritability of protoplasm to certain stimuli. 



Growth. — Organisms subsist on materials taken from the 

 environment. During metabolism a certain portion of the 

 absorbed food products is built up into protoplasm by what is 

 known as intussusception, which means the deposition of new 

 particles of material among those embodying the living sub- 

 stance. Protoplasm is constantly growing from material 

 supplied from the outside but incorporated from the inside. 

 This occurs after undergoing preliminary changes, in chemical 

 and physical form, called digestion. Food in the process of 

 digestion is changed into a form in which it can pass through 

 surfaces of cells and be assimilated. Only a fraction of the 

 digested food is required for anabolic processes. Most of it is 

 oxidized without ever becoming an actual part of living tissue. 

 The immediate cause of increase in size of an organism is the 

 multiplication of individual cells by a process of cell division. 

 When the adult stage is reached, cell division ceases in the body 

 generally. The growth process characterizing the preadult 

 period results from an excess of anabolic over catabolic processes, 

 accompanied by cell multiplication. 



Size and Form. — Another peculiarity of living things is that 

 each kind has a determinate size and form, which is reproduced 

 as a rule generation after generation within rather narrow limits 

 of variation. Lifeless objects may be of almost any size and 

 form as, for example, water may be in the form of a rain drop or 

 a lake; a stone may be a pebble or a mountain. The distinctive 

 size and form of any given species are due to factors that are 

 rooted in the organization of the protoplasm of that particular 

 species. 



Reproduction. — Living things reproduce their own kind. In 

 Protozoa, unicellular animals, reproduction consists in the 



