go 



SYSTEMATIC ZOOLOGY 



as well as in the deep sea. Typically they consist of a central 

 disc from which five arms radiate, broad at their region of 

 attachment to the disc, and tapering toward their tips. In some 

 species there is a much larger number of arms. The aboral 

 surface of the animal is usually more or less covered with spines, 

 and with short, soft, tentacle-like processes, which probably 

 assist in respiration. In one interradius on this surface is the 

 madreporic plate already referred to ; this is often of a bright 



orange color, and its 

 position furnishes an 

 opportunity for nam- 

 ing the arms, the two 

 adjoining the plate be- 

 ing called the bivium, 

 the remaining three 

 the trivium. Further, 

 it will be noticed that 

 if a vertical plane be 

 passed through the 

 madreporic plate and 

 the arm opposite it, 

 this plane divides the 

 starfish into two equal, 

 symmetrical parts, 

 and is therefore the 

 plane of bilateral symmetry of the body. 



At the tip of each arm, on its oral side, is a bright red spot, 

 which is a sort of eye, and when the starfish walks about, the 

 tips of the arms are kept turned up, so that these eyes may be of 

 use. Along each arm, on its oral side, is the ambulacral groove, 

 in which are usually either two or four rows of tube feet (Fig. 79). 

 The ambulacral groove is bounded by rows of well-developed 

 spines. The spines of the starfish are frequently surrounded at 

 their bases by minute pincerlike structures, sometimes sessile, 

 sometimes raised on a stalk, and the jaws of the pincers may be 

 straight, or crossed like the blades of a pair of scissors; these 

 organs are called pedieellarise, and their function is very doubtful. 

 The mouth of the starfish lies in the center of the disc at the 

 point from which the ambulacral grooves diverge, and opens 



FIG. 79. Oral surface of a common starfish, showing 

 tube feet. (Fiom Parker and Hasvvell's Manual.) 



