I 



INTERNAL STRUCTURE OF THE SEA-URCHIN. 93 



AHstotle^s lantern^ which is made up of a great number 

 of calcareous plates, and projects from the peristome into 

 the body cavity. 



5. In contact with the wall of the oesophagus, a small 

 membraneous tube, the stone canal (Fig. 40, a) runs from 

 the madreporic body across the perivisceral cavity to the 

 lantern. 



Fig. 40. 



Fig. 40. — Internal view of the oral half of the corona of Arbacia 

 piinctulata, to show the muscles of the lantern. (Drawn from nature by 

 H. J. Rice, imder the author's direction.) 



n. Stone canal, h. Circular water tube. c. Concentrator muscles of 

 lantern, d. Tendons of lantern, e. Protractor muscles. /. Radulae. 

 g. Kadii. h. Retractor muscles, i. Kadial water tubes, j. Ampullae. 



6. The stone canal joins the circum-oral water tube 

 (Fig. 40, h), which surrounds the oesophagus where itr'^ 

 enters the lantern. 



