ECHINODERMATA 243 
is soft and membranous, with scattered ossicles. The mouth 
opens into an oesophagus, surrounded in the Regulares 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 Regulares 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. 
