ECHINODERMATA 243 



is soft and membranous, with scattered ossicles. The mouth 

 opens into an oesophagus, surrounded in the Eegulares and 

 Clypeastroids by a complicated masticatory apparatus known 

 as Aristotle's Lantern. The oesophagus extends into the inter- 

 radius of the madreporic plate, and opens into an intestine 

 which takes a spiral course, finally opening by the anus, which 

 may be nearly central in position or quite eccentric. The 

 intestine is accompanied by a second tube, the siphon, which 

 may have been pinched off from the intestine, into which it 

 opens at each end. The whole is held in position by numerous 

 strands of connective tissue. 



The body-cavity is spacious and is filled with a fluid in 

 which amoeboid corpuscles float, similar to those found in the 

 water-vascular system. 



The circumoral water- vascular ring lies at the dorsal end 

 of Aristotle's Lantern. The ring gives off in each inter- 

 radius a diverticulum or Polian vesicle, and in each radius a 

 radial vessel which runs along the inner surface of the ambu- 

 lacral plates ; it bears a number of ampullae, which open, as a 

 rule, by two ducts into the tube-feet, these vary much in 

 structure; when suctorial the sucker contains calcifications. 

 In the interradius of the madreporic plate a stone canal, which 

 may be membranous or calcified, passes to an ampulla which 

 opens by the madreporic plate. 



The blood system of Echinoids is still involved in obscurity. 

 There is a circumoral ring adjacent to the water-vascular ring, 

 giving off two vessels which run one on each side of the 

 intestine, and there are probably radial vessels, and one or 

 more vessels accompanying the stone canal. Glandular tissue 

 representing the so-called heart of other forms is developed in 

 the wall of this structure. 



In the Eegulares ten buccal gills are usually found pro- 

 jecting from the peristomial area around the mouth ; these are 

 hollow arborescent diverticula of the coelom, resembling in 

 essential structure the dermal branchiae of the Asteroids. 



There is a circumoral nervous ring situated in the angle 

 between the base of Aristotle's Lantern and the peristome ; 

 this gives off five radial nerves, each of which ends in a sensory 

 prominence of the epidermis, which traverses the ocular plate. 



