Phylum Echinodermata and Related Groups 545 



preserved specimens such as are studied in the laboratory, the color is 

 usually dull orange. In the manner which is typical of this class, 

 the arms are broadly joined to the central disc. The two arms with 

 the madreporite between are called the bivium; the remaining three, 

 the trivium. 



The mouth is located in the central portion of the oral surface. 

 Around the mouth are five groups of spines or papillae. Radiating from 

 the mouth are the five amhulacral grooves, each in the center of one of 

 the arms. The tube jeet are arranged in rows of fours within these 

 grooves. In the living specimens the movements of these feet are 

 easily seen. 



Fig. 178. — External view of the starfish, Asterias. A, Aboral 

 surface; B, oral surface. 



The aboral surface has as its most conspicuous structure the 

 madreporite or sieve plate. In the living specimen this is bright red, 

 but in preserved material it is of the same color as the rest of the 

 body. On this surface are many spines which project from the skeleton. 

 Among them are located tiny papillae, the dermal branchiae, which serve 

 for respiratory exchange. In the living animals these resemble a fuzz 

 over the aboral surface which can be seen to expand and contract. 

 Around the bases of the spines are located the pincerlike pedicellariae 

 (Fig. 180). In the center of the aboral disc is the anus. 



The entire surface of the animal is covered with ciliated epidermis 

 beneath which is the endoskeleton composed of calcareous plates or 

 ossicles. The skeleton itself is composed of small tube-shaped ossicles 

 which are in a leathery body covering. Their arrangement is such 



