Classification , 431 



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, 



pentamerous echinoderms without arms ; the outer covering composed 

 of calcareous 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. Polygordius and Protodrilus. 



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 

 ventro-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- 

 oped somewhat in the arthropodan direction. Lankester thinks their 

 evolution is as follows : 



Group Articulata 



1. Rotifera to Tardigrada 



2. Chaetopoda 



a. Proarthropoda (Peripatus) developing independently. 



b. Crustacea — separate origin from Chaetopoda. 



