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 papule, 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 pedicellarie ; 
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 
