CHARACTERS OP THE SUB-KINGDOMS. 23 



yond the sac for a distance which is not uncommonly 

 equal to many times the length of the latter." (Huxley.) 

 These organs are doubtless used by the animals in killing 

 their prey, as well as in defence. Some of the "jelly 

 fishes' 7 have filaments large enough to penetrate the 

 human skin. 



No organs of circulation or respiration are known to 

 exist. Only in one group is there any trace of a ner- 

 vous system. All of them have distinct reproductive 

 organs. 



This sub-kingdom is divided into two classes : 



1. Actinozoa (Gr. afais, a ray; zoon, an animal). 



2. Hydrozoa (Gr. fmdra, a water serpent ; soon, an 

 animal). 



SUB-KINGDOM VII. Protozoa, 



21. The Protozoa (Gr. protos, first ; zoon, an animal), as 

 the name signifies, is the lowest primary division of the 

 animal kingdom. Most of the animals in this division 

 are so small that they can only be seen by a powerful 

 microscope. Some, however, as the sponges, are of large 

 size. On account of their minute size they are popularly 

 called animalcules. Until recently, they were compara- 

 tively unknown even to the scientific world. Most 

 people are still unaware of their existence. They are 

 essentially aquatic animals, and may be found abun- 

 dantly in any stagnant pond, or in water containing 

 decayed animal or vegetable matter. 



The bodies of the Protozoa are composed of a substance 

 termed sarcode (Gr. sarx, flesh ; eidos, form). It is also 

 sometimes called protoplasm (Gr. protos, first ; plasso, I 

 mould.) This substance is jelly-like, and has been de- 

 scribed by some observers as resembling the white of an 

 e gg y by others it is said to be like a particle of " thin 

 glue." It is mainly composed of albumen, and is without 

 any definite structure. 



Except in one group, the Infusoria, there is no perma- 



