26 TEXTBOOK OF ZOOLOGY 



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

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

 in soil water, and still others as parasites in man and other animals. 

 About 15,000 are known. 



Phylum Porifera (Sponges). — Aquatic metazoans which live at- 

 tached. Most of them are marine. The body is supported by 

 fibrous, calcareous, or siliceous spicules, and the body wall is per- 

 forated 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 cavit.y, and tentacles provided with stinging bodies, 

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



Phylum 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, elongated worms which possess 

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

 sitic. The hookworm, ascaris, and the "horsehair worm" are com- 

 mon representatives. 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 loco- 

 motion. The common representatives are starfishes, sea urchins, 

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

 species. 



Phylum Annelida (Jointed worms). — This group is characterized 

 by segmented body, well-developed body cavity, and nephridia as 

 tubular excretory structures. They live in marine waters, fresh 

 water, and in the soil. The earthworm and leech are well-known 

 examples of the phylum. There are at least 4,500 known species. 



