Echinoderms, Worms. 57 



tinct nervous system, a digestive canal which in the adult 

 is separate from the peritoneal or body cavity (though in 

 the embryo, the body cavity arises as an outgrowth from 

 the primary intestine), a radiating symmetry tending to 

 become bilateral, and a water- vascular system whose 

 little tubular feet or offsets act as locomotory organs. 



The chief sub-types may be tabulated thus 



A. Body stalked at some period of life, ambulacra! 



feet not locomotory = Class I. Crinoidea 

 (mostly fossil). 



B. Never stalked, star-like, with ambulacra as organs 



of locomotion. = Class II. Stellerida. 



a. Arms jointed to the disk, not containing 



viscera = Order I. Ophiuroidea 

 (Brittle Stars). 



b. Arms not jointed, containing viscera 



Order II. Asteroidea (common Star- 

 fishes,). 



C. Never stalked, globular, disk-like or heart-shaped, 



with a continuous test Class III. Echinoidea. 



D. Never striked, elongated, with a soft integument 



containing spicules = Class IV. Holothuroidea. 

 The Echinodermata are all marine, and are never 

 united together into colonies. 



CHAPTER XI. 



SUB-KINGDOM V. VERMES. 



WORMS, though often mean and uninteresting in 

 external appearance, are yet in many respects among 



