THE ANIMAL KINGDOM 437 



the animal along. The ambulacra are also used in capturing and 

 holding prey (Fig. 257). They are important as tactile organs. 

 The fluid in the ambulacral system is moved by cilia on the inner 

 surface of the stone canal and by the contractions of muscles in 

 the walls of the ampullae and the ambulacral feet. In addition 

 to the ambulacral system there is also a blood circulatory system 

 in which blood is circulated by cilia lining the vessels. 



The radial symmetry is impressed on all of the internal 

 organs. The body cavity is a complicated system of spaces, some 

 of which are cut off from the rest, and all containing a fluid 

 similar to that of the ambulacral system. The alimentary canal 

 is usually a complete tube and lies in the largest of these spaces. 

 The nervous system consists of (1) a superficial nerve ring around 

 the esophagus, with radial cords extending into the arms; (2) 

 a deeper oral ring and radiating nerves; (3) an apical system in 

 the aboral wall but not present in all echinoderms. Respiration 

 and excretion are carried on at the surface of the body, which 

 is usually ciliated. Some brittle stars reproduce asexually by 

 fission, but otherwise reproduction is sexual. The sexes are 

 usually separate. While the eggs normally require fertilization, 

 it has been found that they can be stimulated to develop by artifi- 

 cial means, artificial parthenogenesis; for this reason they have 

 been favored material for experimental work along these lines. 

 Ripe echinoderm eggs, when subjected to the effects of hypertonic 

 sea water, certain acids, temperature changes, or mechanical shock, 

 will develop without the intervention of a sperm. 

 CLASS I. CRINOIDEA, Sea lilies and feather stars. 

 Have a nonciliated, cup-shaped body, the calyx, usually 

 attached by a stalk on its aboral side. The oral surface is 

 pointed upward, and contains the mouth, and also the anal 

 opening. The arms, 5 or 10 in number, are usually branched 

 and feathery in appearance, and bear ambulacral grooves on the 

 aboral surface. Crinoids were more abundant in paleozoic 

 times than now, about 2,100 fossil species being known (Pratt). 

 Example: Hathrometra tenella, a feather star. Atlantic 

 Coast, at 150 to 3,000 ft. 

 CLASS II. ASTEROIDEA. Starfishes. Usually have five 

 arms, with open ambulacral grooves on the oral side, through 

 which the ambulacra are extended. Gastric pouches and 

 hepatic caeca extend into the arms. Spines, pincerlike 



