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 tJicy ai-e 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 ail 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 — Wheklworms (Trochelminthes). The Rotifera, or wheel 

 animalcules. Mostly microscopic. 



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