THE STARFISH 27 



The body is composed of calcareous plates, or ossicles, 

 joined to one another by a soft, flexible membrane. Note 

 the many harsh spines all over the body, both above and 

 below. This array of spines is a characteristic of the branch, 

 Echinodermata, although it is not constant. Note that the 

 body consists of a central disk from which radiate five 

 arms. This radial symmetry is universal throughout the 

 Echinodermata. Are the spines ever in rows on the arms ? 

 What is the shape of each arm ? Does every specimen have 

 five arms ? If not, find a possible explanation. The lower 

 surface of the body upon which the mouth is situated is 

 called the oral surface. The upper surface is called the 

 aboral surface, i.e. the surface away from the mouth. 



Note on the aboral surface, between the bases of two arms, 

 a small, circular plate, marked with undulating lines. 

 This is the madreporite. It is the cover to the stone canal. 

 Find the small, soft, filiform processes that project on Hie 

 aboral surface between the calcareous plates. These are 

 the branchice. 



Make a drawing of the aboral surface, showing as many 

 of these structures as possible. 



On the oral surface, in the center, note the mouth. 

 Note the spines projecting over it. Cut these away and 

 find the opening of the mouth. Is it circular or pentagonal ? 

 Note that a- groove runs along the middle of each arm. 

 These are the ambulacral grooves. Each one of these 

 grooves is bordered by two or three rows of movable 

 calcareous spines. 



Note on the sides of each groove two rows of soft white 

 projections, the tube feet. The areas occupied by the tube 

 feet are the ambulacral areas. 



Make a drawing showing these structures. 



