428 GENERAL BIOLOGY 



Class CESTODA ( ). Parasitic flatworms with 



the body differentiated into a head (scolex) and a chain of similar joints 

 (proglottides), the whole being usually regarded as a colony. Tape- 

 worms. 



Phylum NEMATHELMINTHES ( ). Bilat- 



erally symmetrical, triploblastic animals with an elongated cylindrical 

 body covered with a cuticle, with a true body cavity, and a digestive tract 

 with both mouth and anus. Roundworms. 



Phylum ECHINODERMATA ( ). Radially 



symmetrical (with minor exceptions), triploblastic animals with well de- 

 veloped coelom, and usually with five antimeres, spiny skeleton of cal- 

 careous plates, and organs of locomotion known as "tube feet" operated 

 by a water-vascular system. Starfishes, sea urchins, sea cucumbers. 



Class ASTEROIDEA .( ). Free-living, typically 



pentamerous echinoderms with wide arms not sharply marked off from 

 disc and with ambulacral grooves. Starfishes. 



Class OPHIUROIDEA ( ). Free-living, typically 



pentamerous echinoderms with slender arms sharply marked off from 

 disc and no ambulacral grooves. Brittle stars. 



Class ECHINOIDEA ( ). Free-living, pent- 



amerous echinoderms without arms ; the outer covering composed of cal- 

 careous plates bearing movable spines. Sea urchins, sand dollars. 



Class HOLOTHURIOIDEA ( ). Free-living, 



elongated, soft-bodied echinoderms with muscular body wall and tenta- 

 cles around mouth. Sea cucumbers. 



Class CRINOIDEA ( ). Sessile echinoderms 



with five arms generally branched with pinnules, aboral pole usually with 

 cirri, sometimes with jointed stalk for attachment to substratum. Feather 

 stars, sea lilies. 



Phylum ANNELIDA ( ). Triploblastic, bilat- 



erally symmetrical elongated animals with external and internal seg- 

 mentation; coelom usually present; setae common. True worms. 



Class ARCHIANNELIDA ( ). Marine Annelida 



with no setae nor parapodia. 



Class CHAETOPODA ( ). Annelida with 



setae and a perivisceral coelom; marine, fresh-water, or terrestrial in 

 habitat. Earthworms. 



Class HIRUDINEA ( ). Annelida without 



setae, and with anterior and posterior suckers. Leeches. 



Class ONYCOPHORA ( ). Annelida breath- 



ing by means of tracheal tufts, numbering from 10 to 40 per segment in 

 irregular arrangement, with non-jointed papillate legs, nerve cords ven- 

 tro-lateral, and without segmental ganglia, eyes of vesicular, annelid 

 type, skin with chitin. This group is often placed with the Arthropoda, 

 or as a separate phylum Proarthropoda, since its members have devel- 



