A SEA URCHIN 151 
contains in its center the minute anus. Surrounding the peri- 
proct are ten plates, which also bear no spines. Five of these, 
which are larger than the others, are situated at the ends of the 
interrays and are pierced each by a small hole. These plates 
are called the genital plates, and the holes are the external open- 
ings of the genital organs. One of the genital plates is larger 
than the others and is porous; it is the madreporic plate. The 
five smaller of the ten plates which surround the periproct are 
situated at the ends of the rays. They are called the ocular 
plates. Hach is pierced by one or two holes, through which 
project minute pigmented tentacles. Notice that each ray and 
each interray is made up of two rows of plates, so that there 
are twenty rows of plates altogether. As in the starfish, the 
two rays between which the madreporic plate lies are called the 
bivium, the other three, the trivium. 
Exercise 4. Make a drawing of the aboral side of the shell, with 
the spines removed, on a scale of 2, showing accurately 
the boundaries of all the plates. Label the rays, interrays, 
and all the other parts observed. 
The internal organs. Place a fresh or preserved sea urchin in 
a pan of water. Carefully cut away the peristome with scissors 
and remove the shell of the oral body-wall on one side of the 
peristome without disturbing the organs within. Observe the 
following systems of organs: 
The digestive system. This is quite different from the same 
system in the starfish. The mouth opens into the esophagus, 
which passes through the center of the large cone-shaped 
dentary apparatus, which is also, because of its shape, called 
Aristotle’s lantern. This is a complicated structure consisting of 
a number of calcareous plates and muscles which project from 
the mouth into the body-cavity. Study its muscular attach- 
ment with the shell. Note the protractor muscles which pass 
from its upper end to the oral body-wall, by means of which 
