136 AN INTRODUCTION TO ZOOLOGY. 



which the Amceba ("Proteus auimalcule ") has bieen 

 already described as the typical form of the animaL 

 Cell. It is a single cell, furnished with a nucleus and 

 a contractile vacuole : its protoplasm, divided into an 

 inner granular layer called the endoplasm, and an 

 outer clear layer called the ectoplasm, is constantly in 

 movement, throwing out or retracting pseudopodia, 

 and thus altering its shape. 



The Heliozoa (sun-animals) are so called because 

 they have stiff radiating pseudopodia all round, which 

 make them look like a conventional representation of 

 the sun with its rays. They are, as a rule, provided 

 with a silicious (flinty) skeleton. They are fresh- 

 water forms, and have a contractile vacuole, which 

 distinguishes them from the next order. The Ba- 

 diolaria are marine forms, which have a skeleton 

 often very like that of the Heliozoa in appearance. 

 Their skeletons, like those of the Foraminifera, have 

 formed geological deposits : being of a silicious cha- 

 racter, they sometimes form a hard stone, useful for 

 polishing slate. An interesting fact about the Badio- 

 laria is that they are often found to contain coloured 

 unicellular plants (algae), which have been described 

 under the name of "yellow cells." These are be- 

 lieved to be, not parasites, but companions, each 

 member of the partnership deriving some advantage 

 from it. Companionship of this kind is called sym- 

 hiosis (living together). 



The higher types of the Inpusokia are more highly 

 organized than the Ehizopoda. Instead of having a 



