Echinoderms, Worms. 57 



tinct nervous system, a digestive canal which in the 

 adult is separate from the 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 feet 



The chief sub-types may be tabulated thus 



A. Body stalked at some period of life, ambulacral 

 feet not locomotory = Class I. Crinoidea 

 (mostly fossil). 



R 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 stalked, elongated, with a soft integument 



containing spicules = Class IV. Holothuroidea. 

 The Echinodermata are all marine, and are never 

 united together into colonies. 



CHAPTER XL 



SUB-KINGDOM V. : VERMES. 



WORMS, though often mean and uninteresting in 

 external appearance, are yet in many respects among 



