124 LECTURE X. 



will permit of two explanations. In the first and most probable mode 

 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 will return 

 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, Avhich 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 tubular 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, would then pass 

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

 the heart by the vessel which passes from the lantern to it. 



The nervous system consists in the Echinidce, as in the Asferias, 

 chiefly of a chord surrounding the pharynx, and of five trunks ex- 

 tending along the ambulacral interspaces. The pharyngeal ring is 

 an equilateral pentagon in the Echinus, and an oblong pentagon in 

 the Spatatiffus. In the Echinus it is situated close upon the inner 

 side of the apices of the calcareous pyramids which support the teeth ; 

 the ambulacral trunks are flattened, and may be distinguished from 

 the overlying branchial vessels by the connection of the latter with 

 the internal branchiae. Smaller nervous branches are sent off from 

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

 oesophagus. The ambulacral or branchial nerves diminish in size as 

 they proceed, supplying the internal branchiae and the ambulacral 

 tubes ; they finally terminate by penetrating the pore of the ocular 

 plate to gain the base of the red ocellus. 



The generative apparatus of the Echinus consists of five mem- 

 branous sacs, the efi'erent ducts of which perforate five plates, sur- 



