176 INVERTEBRATE ZOOLOGY 



vessel. Interradially, five Polian vesicles communicate with 

 the circular canal and a radial vessel passes along each ambulacral 

 area on the inner surface of the test. In the ambulacral plates, 

 there are two perforations for each tube-foot. Through one of 

 these, the lateral branch of the radial canal passes to the tube-foot 

 and through the other the foot is in communication with its 

 ampulla. In many forms, the ambulacra are aided in the loco- 

 motor process by the highly developed mobile spines which are 

 articulated with the surface of the test and are operated by special 

 muscles. 



One of the most characteristic structures of the echinoid is 

 the Aristotle's lantern. From the oral surface of the animal, the 

 five teeth with which this organ is supplied are visible in the 

 center of the peristome. The main part of the lantern lies within 

 the body cavity. The teeth are at the tips of a set of jaws which 

 are operated by muscle bundles attached to a calcareous frame- 

 work of intricate pattern surrounding the mouth cavity. From 

 the aboral surface of the lantern is given off a short esophagus. 

 This in turn leads into the stomach, a part of which is greatly 

 dilated and flattened and extends almost around the body. The 

 intestine bends backward in the opposite direction from that of 

 the course of the stomach and in the case of the urchins passes 

 to a median dorsal anus, while in the sand dollars it passes along 

 the posterior interambulacrum to an anal opening on or near the 

 margin of the disc. The siphon occurs as a branch from the 

 esophagus which parallels the course of the stomach for some 

 distance, then reunites with it. It seems probable that this 

 heavily ciliated tubule may have a respiratory function and may 

 also be of service in washing refuse from the intestine. 



When a sea urchin is opened for dissection by removal of the 

 aboral wall of the test, the gonads are usually the most conspicu- 

 ous structures first encountered. These are five large masses, 

 interambulacral in position, connected at the aboral pole by a 

 band of tissue termed the genital rachis. From each gonad a 

 gonoduct passes to the opening in the adjacent genital plate. 

 The larva (Fig. 86 ^) which results from the cleavage and later 

 development of the fertilized egg is termed a pluteus. Since the 

 term pluteus is also applied to the larva of Ophiuroidea, the 

 name echinopluteus is frequently utilized. 



Respiration is performed to a considerable extent by the water- 

 vascular system. In some echinoids, only part of the tube-feet 



