A SEA URCHIN 121 



holes, through which project minute, pigmented tentacles. Notice 

 that each ray and each interray is made up of two rows of plates ; 

 thus there are twenty rows of plates altogether. As in the star- 

 fish, the two rays between which the madreporite 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 attachment with the shell. 

 Note the protractor muscles, which pass from its upper end to the 

 oral body wall, by means of which the apparatus can be thrust 

 down and partly out of the mouth; note also the retractor 

 muscles, which pass from the lower part of it to the tall inner 

 projections of the shell. 



Exercise 5. Draw a diagram showing the dentary apparatus in the 

 body and its muscular attachment to the shell. 



The oesophagus, after leaving the dentary apparatus, passes to 

 the elongated intestine ; this lies close to the body wall, to which 

 it is attached by means of a mesentery. Carefully follow the 

 intestine, breaking away the wall if necessary, as it winds around 

 the inner surface of the shell. Note the intestinal siphon, a short 

 tube which lies alongside the intestine, joining it at both ends. 

 From the intestine a short rectum passes to the anus. In mak- 

 ing this dissection keep the animal immersed in clear water ; 



