90 STARFISH, SEA URCHLN, BRITTLE STABS 



nhoral surface. The opposite side is called the oral sur- 

 face (Fig. 43). In the center of the oral surface is an 

 opening, the mouth. Radiating from the mouth are five 

 grooves, one along the median line of the oral side of 

 each arm. These are the ambulacral grooves. The 

 whole body is beset with many stiff spines. Bordering 

 each of the ambulacral grooves there are two or three 

 rows of spines that are movable. These are the ambulacral 

 spines. On the aboral surface, between the bases of two 

 arms, is a small circular plate, the madreporite, which is 

 marked by a number of fine, straight, or wavy ridges and 

 perforated with many minute openings. 



The tube feet. — In each of the andjulacral grooves there 

 are two double rows of soft, fle.Nible, and cylindrical bodies, 

 the tube feet. Each tube foot is a hollow cylinder, ends with 

 a sucker, and can be greatly extended or contracted. These 

 are the organs of locomotion. 



Structure of the body walls. ^Tlie body walls consist 

 of three layers: an outsitle layer of thin epidermis that 

 covers the whole body; a middle and much thicker layer, 

 ike mesoderm, and a delicate inner layer, the ca^lomic epithe- 

 lium. The latter not only lines the whole body cavity, 

 but forms an outside covering for the internal organs. 



Skeleton. — Tlie body of the starfish is inclosed in a 

 hard, rough integument, the skeleton, consisting of many 

 small, irregular, calcareous plates, termed ossicles. The 

 ossicles are loosely joined together by a connective tissue 

 so that the skeleton is more or less flexible and a certain 

 amount of movement is permitted. The skeleton is be- 

 neath the epidermis and is developed from the mesoderm. 



In the spaces between the ossicles, on the dorsal surface, 

 there are several minute pores in the connective tissues. 



