CHAPTER LXXXVII 

 KINDS OF ANIMALS 



504. The classification of animals. Distinctions between 

 higher and lower among animals are based on the same con- 

 siderations as those among plants, namely, complexity of 

 structure and specialization of functions. The most striking 

 division among animals is that between the vertebrates (animals 

 having a backbone) and the invertebrates. The latter group 

 is made up of many diverse types that have little in common 

 except the fact that they are animals. 



505. The main groups of animals. The chief groups of 

 animals are indicated in the following outline : 



DIVISION I PROTOZOA. The simplest animals ; body of one cell. (Ex- 

 amples. Ameba, Paramecium, Vorticella, Plasmodium of malaria.) 



DIVISION II PORIFERA (" pore-bearing " animals). This includes all the 

 sponges. 



DIVISION III CCELENTERATA. Radially symmetrical animals having a 



single cavity in the body ; all aquatic, mostly marine. 

 Class i Hydrozoa. (Examples. Fresh-water hydra, certain small 



jellyfish.) 



Class 2 Actinozoa. (Examples. Sea anemones, most corals.) 

 Class 3 Scyphozoa. (Examples. Most of larger jellyfish.) 



DIVISION IV FLATWORMS (Platyhelminthes). (Examples. Tapeworm, 

 liver fluke, planarians.) 



DIVISION V ROUNDWORMS (Nemathelminthes). (Examples. Hookworm, 

 trichina, thorn-headed worm.) Many of these animals are dangerous 

 parasites on man or on domestic animals. 



DIVISION VI WHEELWORMS (Trochelminthes). The Rotifera, or wheel 

 animalcules. Mostly microscopic. 



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