INTRODUCTION 23 



The secondary groups are phyla, and they in turn are divided into 

 classes. The principal groups subordinate to the class are order, 

 family, genus, and species. Later in the book there is a chapter 

 devoted to the classification of animals but the principal phyla are 

 listed and briefly described here : 



Phylum Protozoa. — Individuals consist either of a single cell or of 

 aggregates of cells, by each of which are performed all the essential 

 functions of life. They are mostly microscopic in size and largely 

 aquatic in habit. Some live in the ocean, some in fresh water, and 

 still others as parasites in man and other animals. About 15,000 are 

 known. 



Phylum Porifera (Sponges). — Mostly marine aquatic metazoans 

 which live attached! The body is supported by fibrous, calcareous, 

 or siliceous spicules, and the body wall is perforated by many pores. 

 There are approximately 3,000 known species. 



Phylum Coelenterata (Jellyfish).— All are aquatic and most of 

 them are marine. They possess radial symmetry, a single gastro- 

 vascular cavity, and tentacles provided with stinging bodies, nemato- 

 cysts. The described species number at least 4,500. 



Phylimi Ctenophora (Sea Walnuts or Comb Jellies).— Free swim- 

 ming, delicate, marine animals that possess biradial symmetry. They 

 are triploblastic and hermaphroditic. Less than one hundred species 

 are known, and twenty-one of these are American. 



Phylum Platyhelminthes (Flatworms). — These are flat, unseg- 

 mented, bilaterally symmetrical, triploblastic worms. ''Flame cells" 

 are characteristic excretory structures. These animals may be free- 

 living or parasitic. Tapeworms, liver flukes, and the free-living, 

 aquatic Planaria are commonly known. Approximately 6,500 species 

 have been described. 



Phylum Nemathelminthes (Threadworms or Roundworms). — Un- 

 segmented, bilaterally symmetrical, enlongated worms which possess 

 both a mouth and an anus. Some are free-living, others are parasitic. 

 The hookworm, ascaris, and the ''horsehair worm" are common rep- 

 resentatives. About 3,500 species are known. 



Phylum Echinodermata. — Marine animals which have a spiny skin 

 and the body wall usually supported with calcareous plates. They are 

 radially symmetrical and have tube feet as organs of locomotion. 

 The common representatives are starfishes, sea urchins, sea cucum- 

 bers, and sea lilies. There are about 4,500 known living species. 



