XIV] 



ECH1NOIDEA 



353 



of digestion were accumulated in them. A so-called "blood-ring" 

 of similar character surrounds the oesophagus just above the water 

 vascular ring and into this the two "vessels" open. A similar ring 

 has been described in Asteroidea and Ophiuroidea : in some species 

 of the former class a tract of similar substance appears to run down 

 the arm just above the nerve-cord in the septum separating the two 

 perihaemal canals, and the name of these canals (Gr. Tre/n, around ; 

 at/x, blood) has been suggested by this circumstance. 



Breathing, as one might expect, is carried out wherever the 

 body-wall is thin enough to allow the oxygen to diffuse through, 



Fm. 166. Oral field of Echinus esculentus. Magnified. From Kiikenthal. 

 1. Ambulacrum with tube-feet and spines. 

 Aristotle's Lantern. 4. Buccal tube-feet, 

 mouth, the peristonie. 



2. Gill. 3. Teeth of 



5. Soft membrane around 



that is to say by the tube-feet and by tne peristomal membrane. 

 The tube-feet, as in the star-fish, are provided with large ampullae 

 which project freely into the great spacious body-cavity. Oxygen 

 thus taken into the fluid filling the tube-feet can be passed into the 

 body-cavity through the ampullae, and there is a curious arrange- 

 ment to facilitate this. Where the tube-foot passes through the 

 s. & M. 23 



