A SEA URCHIN 141 



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. Each is pierced by one or two minute holes through 

 which project minute pigment eyes. 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 oesophagus, 

 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 



