6 OUTLINES OF NATURAL HISTORY. 



the world around it ; and it reproduces its like. All 

 animals discharge these three sets of functions, but the 

 higher animals have much more complicated relations 

 with the world outside than the lower. The study of 

 the way in which these functions are discharged by dif- 

 ferent animals, belongs to the science of Physiology, and 

 will not be discussed here. In what follows, therefore, 

 we shall simply consider the form or structicre of the more 

 important types of animal life. 



CHAPTER IL 



CLASS EHIZOPODA. 



The animals belonging to this class are mostly very 

 minute, and none of them, except the Sponges, can be 

 said to be familiarly known. Owing to their complexity 

 of structure, comparatively speaking, it will not be ad- 

 visable to take one of the Sponges as the type of this 

 class. We shall therefore select as an example the 

 little animalcule known as the Amoeba. 



The Amoeba, or " Proteus- Animalcule " (fig. i), is an 

 inhabitant of most collections of stagnant water, especially 

 where decaying vegetable matter is present ; and it is 

 microscopic in its dimensions. It derives both of its 

 names from its wonderful power of altering its shape 

 (Greek, amoibos, changing ; Latin, Proteus, a sea-god who 

 had the faculty of assuming difi'erent shapes). This 

 power it owes to two circumstances. In the first place, 

 the entire body is composed of a soft gelatinous sub- 

 stance, which has very little cohesion, and which can 

 readily flow in different directions. In the second place, 

 the animal has the power of thrusting out thick, blunt, and 

 finger-shaped processes of its body-substance, which may 



