(Opiu'opiioJ 



_ cucumber! 



P' ITbyone) /j .'> 



Sea uichin 

 ^ {Arbacia) 



Sea mouse 

 iAphrodjf) 



PHYLUM VI WHEELWORMS (Trochelminthes, "wheel worms"). Minute 

 "worms" with front end of body cihated and hind end usually forked; the 

 beating cilia on the rotifers give impression of one or more revolving wheels; 

 abound in stagnant water. 



PHYLUM VII ECHINODERMS ("spiny-skinned"). Radially symmetrical 

 marine animals; usually with calcareous spines in skin and with well-developed 

 water-tube system (see illustration, p. 230). 



CLASS 1 ASTEROIDS. Starfish. 



CLASS 2 OPHIUROIDS. Brittle stars. 



CLASS 3 ECHINOIDS. Sea urchins. 



CLASS 4 HOLOTHUROIDS. Sea cucumbers. 



CLASS 5 CRINOIDS. Sea lilies. 



PHYLUM VIII ANNELIDS ("ringed"). Cylindrical worms with segmented 

 bodies; red blood in a closed circulatory system; comparatively highly developed 

 nervous and sensory system. The two most important classes are represented by 

 earthworms and sandworms, which have bristles, or setae; and leeches, which are 

 without bristles and have a sucker at each end. 



PHYLUM IX ARTHROPODS ("jointed legs"). Have jointed limbs, a hard 

 outer covering, the exoskeleton, and segmented bodies; jaws work sidewise. 



CLASS 1 MYRL^PODS ("thousand legs"). The millepedes, with incon- 

 spicuous antennae and two pairs of legs on each segment; and the centipedes, 

 with conspicuous antennae and one pair of legs on each segment. 



CLASS 2 CRUSTACEANS ("crusty shells"). Head and thorax fused into 

 a cephalothorax; five or more pairs of legs; water-breathers; antennae. 

 Examples, crayfish, crab, shrimp, barnacle, sow-bug, lobster (see illustrations, 

 pp. 173, 359, 391, 420 and 461). 



CLASS 3 ARACHNIDS (spiders, "spinners"). Four pairs of legs; air- 

 breathers; no antennae; a cephalothorax. (The horse-shoe crab is an excep- 



Sow bug 



693 



