352 ECHINODERMATA [CH. 



body-wall has little more consistence than the jelly of an Aurelia, 

 and readily degenerates into slime. The ground substance has 

 remained so fluid that it is still traversed by amoebocytes which 

 carry excreta to the interior. In Echinoidea along two tracts, one 

 situated on the same side of the oesophagus as the stone-canal and 

 the other on the opposite side, the jelly intervening between the 

 inner wall of the coelom and the oesophagus has undergone the first 

 stage in the change to a blood-vessel. The fibres are scantily 

 developed and the amoebocytes are present in immense numbers, 



3 



FIG. 165. View of Sea-urchin, with part of the shell removed to show the 

 course of the alimentary canal. From Leuckart, after Cuvier. 



1. Mouth surrounded by five teeth (displaced). 2. Lantern of Aristotle. 



3. Oesophagus, coiled intestine and rectum. 4. Ovaries with oviducts. 

 5. The siphon. 6. "Blood "-ring. 7. Fold of peritoneum supporting 

 genital rachis. 8. "Blood "-vessel accompanying intestine. 9. Ampullae 

 at base of tube-feet. 



whilst the ground substance has become more fluid and probably 

 contains proteids, since it stains with carmine like protoplasm. 

 These, tracts are termed dorsal and ventral " blood "-vessels. The 

 dorsal vessel is on the side next the stone-canal. These tracts 

 have not the form of tubes, but are networks of irregular passages 

 devoid of proper walls. They accompany the alimentary canal 

 throughout most of its course and it seems as if the products 





