INTRODUCTION 17 



dages; setae in the skin. Coelom opens to exterior by dorsal pores 

 and ventral nephridiopores. Alimentary canal well developed and 

 usually specialized. Nervous system consists of two dorsal ganglia 

 and a ventral chain of ganglia. (Examples — earthworm, leech.) 



Phylum Echinodermata (3,000 species). — Triploblastic (three 

 layered) with a calcareous exoskeleton in plates or as spicules; 

 larvae bilaterally symmetrical but adults radially symmetrical. 

 The coelom is well developed; slow locomotion facilitated by water 

 vascular system; all marine; never bud to form a colony. (Ex- 

 amples — starfish, sea urchin, and brittle star.) 



Phylum Nemathelminthes (1,500 species). — Unsegmented with 

 an elongate cylindrical body covered with tough cuticle; tubular 

 digestive tract with mouth and anus; coelom present; paired ex- 

 cretory organs and tubular gonads; nerve ring and associated gan- 

 glia; many parasitic. (Examples — hookworm, eel-worm, trichina.) 



Phylum Platyhelminthes (4,600 species). — Unsegmented flat- 

 tened worms, bilaterally symmetrical and with three distinct layers 

 (triploblastic). Free living forms have a gastro-vascular cavity 

 with no anus, while the degenerate parasitic forms lack a digestive 

 cavity. (Examples — liver fluke, tape worm, and planaria.) 



Phylum Coelenterata (4,500 species). — Radially symmetrical 

 with two cellular layers, and a non-cellular mesoglea; single gastro- 

 vascular cavity or Coelenteron; formerly called Zoophytes. Sting- 

 ing cells or nematocysts in the body wall. (Examples — corals, sea 

 anemones, jelly fishes and hydroids.) 



Phylum Porifera (2,500 species). — Bodies of sponges consist 

 of a mass of connective tissue with two-layered (diploblastic) body 

 wall penetrated by canals or pores. All are aquatic, mostly marine. 

 Radially symmetrical; skeleton of spicules usually supports the body 

 wall. (The common bath sponge is an example.) 



Phylum Protozoa (10,000 species). — Although it would seem 

 that in some respects sponges might be considered colonial Protozoa, 

 we must distinguish the latter from all Metazoa by their charac- 

 teristic of being complete single celled animals, without true tissues. 

 They are mostly so small as to be visible only with the aid of a 

 microscope; many species are colonial; many are parasitic. 



PHYLUM PROTOZOA. 

 Class I. Sarcodina. 

 Class II. Mastigophora 

 Class III. Infusoria. 

 Class IV. Sporozoa. 



