PHYLUM ECHINODERMATA 



227 



constitute the hivium. The other three arms and their adjacent por- 

 tions of the central disc compose the trivium. On the oral side sur- 

 rounding the mouth is a perioral membrane or peristome. An amhu- 

 lacral groove, containing rows of tube feet, radiates from this along 

 the oral side of each arm. A reddish pigment spot in the end of 

 each arm is called an eye. The spines are longer and stronger around 

 the mouth and along the margins of the ambulacral grooves than 

 elsewhere. 



Fig. 122. — The ochre starfish, Pisaster ocJiraceus, an abundant form along- the 

 Pacific coast (XVi). (Johnson and Snook, Seashore Animals of the Pacific Coast, 

 published by The Macmillan Company.) 



Internal Anatomy 



The body wall is relatively strong and hard without being per- 

 fectly rigid. This condition is due to the presence of the calcareous 

 skeletal plates throughout, which are bound together by connective 

 tissue and muscular fibers. These plates are often called ossicles. 

 They lie in a flat position in the aboral portions of the body wall. 

 The skeleton of the ambulacral grooves consists of four rows of 

 articulated, oblong ossicles in each arm. These ossicles are ar- 



