INTRODUCTION 



one knows exactly how many species have 

 been described in any phylum. 



Synopsis of ihe phyla 



Our survey of the animal kingdom will 

 treat only the 11 most important phyla out 

 of the 20 or more which compose it. These 

 11 phyla include about 98 per cent of all 

 species of animals. The estimates of num- 

 bers of living species are from authorities, 

 but new forms are being named all the time, 

 so all figures must be regarded as tentative. 



1. Phylum Protozoa 



These animals (30,000 species) are mostly 

 microscopic in size, and each consists of a 

 single cell or of simple colonies of cells. 

 They live in fresh water, in the sea, in the 

 soil, and in other moist places, and as para- 

 sites on or within the bodies of other ani- 

 mals. Some of them, such as the malarial 

 organisms and the dysentery amoeba, are 

 important in our study because they pro- 

 duce disease in man. 



2. Phylum Porifera 



The sponges or pore bearers (5000 spe- 

 cies) live only in water— most of them in 

 salt water. The body wall is perforated with 

 many pores and is usually supported by a 

 skeleton of spicules of calcium carbonate, 

 silica, or spongin. The commercial bath 

 sponge consists of spongin. 



3. Phylum Coelenterata 



Most of the coelenterates (10,000 species) 

 also live in salt water. They are the hydroids, 

 polyps, jellyfishes, sea anemones, and corals. 

 A common fresh-water type is the hydra. 

 Coelenterates are radially symmetrical, pos- 

 sess single gastrovascular cavities, and are 

 provided with peculiar stinging capsules 

 called nematocysts. 



4. Phylum Ctenophora 



The ctenophores (100 species) are mostly 

 free-swimming marine animals that resem- 



ble the coelenterate jellyfishes and are com- 

 monly called sea walnuts or comb jellies. 

 They are biradially symmetrical. 



5. Phylum. Platyhelminthes 



These are wormlike, unsegmented, bi- 

 laterally symmetrical animals (10,000 spe- 

 cies) known as flatworms. Certain tape- 

 worms and flukes are serious parasites of 

 man and lower animals. Other flatworms 

 live on land, in the sea, and a few live in 

 fresh water, including planaria, the type usu- 

 ally studied in general zoology. 



6. Phylum. Nemathelminthes — Nematodes 

 The threadworms or roundworms (12,- 



000 species) are likewise unsegmented and 

 bilaterally symmetrical. They possess both a 

 mouth and an anus. Many of them are free- 

 living, that is, they live in salt water, fresh 

 water, or in the soil; but others are parasites 

 in plants and animals, such as the hook- 

 worm, roundworm, and trichinella of man. 



7. Phylum. Annelida 



The body of an annelid consists of a row 

 of little rings or segments; hence the mem- 

 bers of this phylum (13,500 species) are 

 known as segmented worms. The earthworm 

 and leech are common representatives. Salt 

 water, fresh water, and the soil serve as habi- 

 tats. 



8. Phylum Arthropoda 



The joint-footed animals belong to this 

 phylum (875,000 species); they are about 

 three times as numerous in species as all 

 other animals. The principal groups of 

 arthropods are the crustaceans, including 

 the lobsters, crayfishes, crabs, and barnacles; 

 the centipedes and millipedes with their 

 many pairs of legs; the insects, such as but- 

 terflies, bees, beetles, bugs; and the arach- 

 noids, represented by spiders, scorpions, 

 mites, and ticks. 



9. Phylum Mollusca 



Snails, slugs, clams, and oysters are com- 

 mon mollusks; others are known as squids. 



