54 BRANCH ECHINODERMATA 



The larva is bilateral/ while the adult is radial, the develo]mient 

 being complex. 



Generally the young shift for themselves, but cases are 

 recorded of broods being cared for by the female echinoderm 

 in a pouch on the dorsal surface. 



CLASS I. ASTEROIDEA 



To this class belong the starfishes, with their central disks 

 and varying number of rays, five being the typical number. 

 They live along rocky seacoasts. Fresh water kills them. 

 The common starfish (Aste'rias vulga'ris) is abundant along the 

 Atlantic coast, especially in the vicinity of oyster-beds, to 

 which they do much injury by devouring the oysters. Star- 

 fishes are found also on the Pacific coast from Sitka to southern 

 California. They are said to devour small fishes as well as 

 crabs. 



The body wall is composed of a thick leathery substance m 

 which is embedded a great number of calcareous ossicles 

 (12,000 by estimation), many of which are armed with spines 

 for protection. Between the spines on the aboral surface are 

 soft stalked projections ending in pinchers, called pedicella'rice, 

 with which it cleanses the surface of the body and protects 

 itself from parasites. 



The alimentary tube extends from the oral to the aboral 

 surface. It consists of a mouth, a short esophagus, and a large 

 sac-like stomach, which is five lobed and fills most of the disk. 

 (See Fig. 41, p. 52.) The stomach is eversible and is furnished 

 with muscles for withdrawing it. From the pyloric, or upper, 

 division of the stomach the cmca extend, a pair into each arm. 

 These caeca secrete much fluid, which is emptied into the 

 pyloric portion of the stomach and used in chgesting the food. 

 From the stomach a short conical intestine extends upward 

 to the aboral surface. The aboral opening from the intestine is 

 not exactly in the center of the disk and is often difficult to find. 

 In a few forms it is wholly obliterated. 



Locomotion. — The arms are somewhat flexible, and, aided by 

 their tube-feet, ^ enable the starfishes to move slowly along in 



1 Hertwig's "Manual of Zoology," Kingsley, p. 331. 



^ See text, Water-vascular System of Echinoderms, p. 52. 



