142 GENERAL BIOLOGY 



Class 1. Asteroidea. Star-shaped forms, the arms not 

 sharply marked off from disc but with an am- 

 bulacral groove from which tube-feet project. 

 Starfish. 



Class .2. Ophiuroidea. Star-shaped forms, the arms 

 sharply marked off from disc and without am- 

 bulacral groove. Brittle stars, serpent stars. 



Class 3. Echinoidea. Spheroidal or discoidal forms with 

 a shell of closely fitting plates and with movable 

 spines. Tube feet project from five rows of pores 

 on the shell. Sea urchins. 



Class 4. Holothuroidea. Elongated worm-like echino- 

 derms with muscular body wall containing scattered 

 plates. Contractile tentacles about the mouth, 

 and tube feet in the form of papillae. Sea cu- 

 cumbers. 



PHYLUM PLATYHELMINTHES. Flattened, bilaterally sym- 

 metrical, worm-like animals with an excretory 

 system of branched canals containing flame cells. 

 Anus is not present, and coelom not developed. 



Class 1. Turbellaria. Free living Platyhelminthes with 

 a ciliated ectoderm and a centrally located muscular, 

 protrusible proboscis. Planaria. 



Class 2. Trematoda. Parasitic forms with unseg- 

 mented, flattened body. Anteriorly placed mouth 

 and one or more ventral suckers. Liver fluke. 



Class 3. Cestoda. Elongated, and usually segmented, 

 Platyhelminthes without mouth or digestive tube. 

 Suckers or hooks at one end, and a complete re- 

 productive system in each mature segment. Tape 

 worms. 



PHYLUM NEMATHELMINTHES. Elongated, cylindrical or 

 thread-like worms with unsegmented body. Ter- 



