viii ECHINODERMATA BLOOD VASCULAR SYSTEM 449 



where the oral nervous system still lies in the epithelium, 

 the epineural canals are wanting. This fact is in harmony with 

 what we know of the formation of the epineural canals, which, as has 

 been proved in the Ophiuroidea, arise through the sinking of the nerve 

 trunks below the surface the nerve trunks rising ontogenetically 

 in the epithelium. These originally epithelial trunks become covered 

 by two lateral integumental folds, which finally grow together over 

 them in such a way that between them and the integument, which 

 now forms a continuous cover over them, a cavity, the epineural canal, 

 remains. The Synaptida also have no epineural canals. This is 

 probably connected with the special way in which their (subepithelial) 

 nerve trunks develop ontogenetically. 



An epineural circular canal is wanting in the Holothurioidea, and, in the 

 Ecliiiioidca, the circular canal is not in communication with the radial epineural 

 canals. A small epineural (periambulacral) space occurs in connection with the 

 development of a circular ganglion at the base of each tentacle in the Ophiuroidea. 



X. The Blood Vascular or Laeunar System. 



Within the connective tissue of various parts of the body in most 

 classes of the Echinodermata there occurs a strongly developed system 

 of very small spaces OP lacunae, which open into one another, and 

 sometimes form at the surface of different organs a fine, close, membrane- 

 like network of lacunae. In other cases, they coalesce to form bundles 

 of canals, running in definite directions, and anastomosing with one 

 another. This lacunar system was formerly universally called a blood 

 vascular system, and may still be allowed to retain this name, 

 although a regular circulation in definite directions of the fluid 

 it contains has in no single case been demonstrated. 



The intercommunicating laeunse, of which the blood vascular 

 system consists, have no walls of their own, and no endothelial 

 lining, and their arrangement in networks or plexuses, which are 

 sometimes spread out flatly, and at others thickened into " vas- 

 cular trunks," is entirely confined to the Echinodermata. 



A localised propelling apparatus is wanting. What was for- 

 merly called the heart has nothing to do with the blood vascular 

 system, but is the axial organ. 



The fluid (blood) contained in the blood vascular system re- 

 sembles that in the body cavity and in the water vascular system, 

 but contains much more albumen in solution. In sections of fixed 

 and stained animals, a vessel can easily be distinguished from the 

 other almost empty cavities of the body by the considerable quantity 

 of coloured coagulum contained in its spaces. The solid constituents 

 floating in the blood are the same as in the body cavity and in the 

 ambulacra! vascular system. 



Leaving altogether on one side the Ophiuroidea and the Asteroidea, 

 in which the existence of a blood vascular system is still doubtful, 

 VOL. II 2 G 





