ECniNODERMATA. 207 



the heart transmits arterial blood to the artery proceeding to the 

 lantern, and from its arterial ring to its soft parts, to the pharynx 

 and to the buccal membrane. From these parts the blood returns 

 into the venous ring of the lantern, and thence into the intestinal 

 vein, where, mingling with the venous blood from the intestine, it is 

 conveyed to the annular vessel of the rectum, which also receives the 

 venous blood of the ovaria. The blood thence passes into the five 

 trunks which represent the branchial arteries. These distribute the 

 blood over the internal gills, or bases of the tube-feet, where it 

 acquires the arterial character. Thus changed, the blood returns by 

 the branchial vein into the arterial ring of the anus, whence it is dis- 

 tributed in part to the ovaria, and the remainder by the intestinal 

 artery to regain the heart. In this view the vessel called by Tiede- 

 mann the intestinal artery performs the office of a vein. 



According to the second explanation, the heart transmits the 

 arterial blood by the intestinal artery to the oesophagus, intestine, 

 and rectum, and then supplies the ovaria, and perhaps also the 

 membrane of the shell. The venous blood collected into the in- 

 testinal vein is poured into the anal venous ring, which receives the 

 ovarian veins, and distributes the blood through the five branchial 

 veins : these will disperse it over the branchial sacs, where it will be 

 oxidized. Thus changed, the. blood returns by the branchial vessels 

 towards the auricles, and would be continued by their apertures 

 into the vessel of the internal oblique ligament, w^ould then pass 

 along the pharynx, gain the arterial circle of the lantern, and re-enter 

 the heart by the vessel w^hich passes from the lantern to it. 



The circular vessel of the chylaqueous system surrounds the gullet 

 at the base of the lantern external to the arterial circle, between it 

 and the nervous ring. With the chylaqueous ring are connected the 

 Polian vesicles and the sand-canal — here a membranous tube. 

 From the circular canal five vessels extend along the ambulacral 

 spaces to supply the system of tube-feet. The chylaqueous has no 

 connection with the proper vascular system. 



The nervous system consists in the Echinidce^ as in the Asterias, 

 chiefly of a delicate chord surrounding the pharynx, immediately 

 external to the chylaqueous ring, and of five trunks extending along 

 the ambulacral interspaces. The pharyngeal ring is an equilateral 

 pentagon in the Echinus, and an oblong pentagon in the Spatangus. 

 In the Echinus the ambulacral nervous trunks are flattened, and may 

 be distinguished from the vessels by the connection of the latter with 

 the internal branchiae. Smaller nervous branches are sent ofi" from 

 each arch of the pentagon to the inter- pyramidal muscles and the 

 oesophagus. The ambulacral or branchial nerves diminish in size as 



