Survey of the Animal Kingdom 303 



Class 2 — Ophiuroidca (of i u -roi' de a) (Gr. ophis, snake; oura, tail; 

 eidos, resemblance). — These are typically free-living, five-rayed (pentamerous) 

 types with the five flexible rays or arms sharply marked off from the central disk. 

 There are no caeca and reproductive organs in the arms. There are no am- 

 bulacral grooves. The body is somewhat flattened. There is no anus. The 

 madreporite plate is on the dorsal upper surface. The tube feet are modified 

 and serve only as tactile organs. 



Examples: Brittle or serpent star (Fig. 103) and basket star. 



Fig. 108.- — Purple sea urchin (Arbacia punctulata) of the phylum Echinoder- 

 mata, class Echinoidea, from the oral or under side. Note the five sharp white 

 teeth (Aristotle's lantern) in the center. (From Coe: Echinoderms of Connecti- 

 cut, State Geological and Natural History Survey of Connecticut, Bulletin 19.) 



Class 3 — Echinoidea (ek i -noi' de a) (Gr. echinos, hedgehog; eidos, resem- 

 blance). — These are free-living types but may be sedentary. There are no free 

 arms or rays, the space between them being more or less filled in. The test or 

 skeleton is composed of twenty columns of firmly united calcareous plates bearing 

 movable spines. These include five pairs of ambulacral rows (perforated for the 

 exit of tube feet) and fiv^e pairs of interambulacral rows of plates. 



Examples: Sea urchin (Figs. 108 and 109), sand dollar (Fig. 110), and heart 

 urchin. 



Class 4 — Holothurioidea (hoi o thu ri -oi' de a) (Gr. holos, whole; thurios, 

 rushing). — These echinoderms have soft, elongated, ovoid, muscular bodies with 



