THE SUB-KINGDOMS. 105 



possess characters in common, which characters are not 

 shared by other cLasses. By attending to these general 

 characters, which belong to several groups, we can ar- 

 range the classes of the Animal Kingdom into six larger 

 divisions, which are called " sub-kingdoms " (Latin, sub, 

 under), because they are the primary sections into Avhich 

 the whole kingdom may be broken up. The following 

 gives briefly the characters of each sub-kingdom and the 

 classes contained in it — two or three small classes, not here 

 considered, being omitted : — 



SUB-KINGDOM I. PROTOZOA. 



The sub-kingdom Protozoa (Greek, protos, first ; zoon, 

 animal) derives its name from the fact that it comprises 

 the lowest forms of animal life known to the naturalist. 

 It includes the two classes of the Rhizoiwda and Infusoria, 

 which exhibit the following common characters : — The 

 body exhibits no distinct rings or segments, and the walls 

 of the body do not enclose any definite cavity which 

 could be properly termed a " body-cavity." They have 

 either no proper digestive system, or, at most, a mouth 

 and short gullet. There is no nervous system, and the 

 only representative of a blood-circulatory system is to be 

 doubtfully found in one or more little contractile cham- 

 bers, which are not universally present. Examples of the 

 sub-kingdom are the Amoeba, the Sponges, and Paramoe- 

 cium. 



SUB-KINGDOM II. CCELENTEKATA. 



The name of Coelente^'ata or "hollow-entrailed " animals 

 is applied to this sub-kingdom because the creatures in- 

 cluded in it have a body-cavity which communicates with 

 the world outside through the mouth (Greek, hoilos, 

 hollow ; enteron, intestine). The sub-kingdom includes 

 the two classes of the Hydrozoa and Actinozoa, and is 

 defined by the following characters : — The body is com- 



