142 INVERTEBRATE ZOOLOGY 



the rays is unequal. This is due to the fact that starfishes 

 often lose one or more of their rays by accident; the missing 

 member is soon replaced by a new ray, but while it is growing 

 out it will be shorter than the others. The spaces between 

 the rays are called interrays. In the center of the under surface 

 of the disc is the mouth; hence this surface of the animal is 

 called the oral surface. Its upper surface is called the aboral 

 surface. 



In the aboral surface of the disc notice the red madreporic plate 

 (in preserved specimens it may have lost its color and be white). 

 Examine it on the dried specimen with the aid of a hand lens or 

 the low power of a compound microscope and notice its porous 

 structure. In the aboral surface is also the anus ; it is a very 

 small opening and will be difficult or impossible to see in the 

 specimens at hand. Note the short fixed spines covering the 

 entire aboral surface. Each one is a part of a small calcareous 

 plate buried beneath the integument. The entire body-wall of 

 the animal is made up largely of these plates, which give 

 it its stiffness. The plates are not, however, connected with 

 one another except by muscles and connective tissue, and the 

 animal's arms are, consequently, flexible and freely movable. 

 Demonstrate this fact with your specimen. In the dried animal 

 this flexibility no longer appears, as the entire body-wall has been 

 rendered rigid by the drying. In the soft places between the 

 plates note the delicate tubular projections of the integument; 

 they are the contractile papulae, and are organs of respiration and 

 excretion and possibly also of sensation. With the aid of a 

 hand lens find, around the base of each spine, the pedicellariae ; 

 these are minute pincer-like organs of somewhat uncertain func- 

 tion, but which probably aid in keeping the surface of the 

 animal free from particles of dirt and from minute organisms 

 which might be harmful. 



The two arms which enclose the madreporic plate between 

 their bases are called the bivium; the remaining three, the 





