ECHINODERMATA. 171 



in absorbing nourishment from the intestine, and conveying it 

 to the vascular circle around the oesophagus, from which the 

 arteries are given off to supply the whole body. These arte- 

 ries are, 1st, a long vessel to the intestine, which runs along 

 its whole length, and anastomoses freely with the branches 

 of the intestinal vein. Sndly, Five arteries to the parts con- 

 nected with the mouth. Srdly, Five dorsal arteries which run 

 along the interior of the shell, between the ambulacral rows 

 as far as the anal orifice, at which point each dorsal artery leaves 

 the osseous box, through an aperture specially provided for its 

 exit, and, arriving upon the outer surface of the shell, supplies 

 the soft external membrane, and in some species may be traced 

 back again between the rows of ambulacral suckers as far as the 

 mouth. These dorsal arteries, like the corresponding vessels in Aste- 

 rias, supply the vascular origins of the innumerable protractile feet. 



(210.) We found in the star-fish that respiration was provided for 

 by the free admission of the external element into the interior of the 

 body ; and in Echinus the aeration of the blood is effected in an 

 equally simple manner. The sea-water is copiously admitted into 

 the peritoneal cavity by a set of membranous tubes provided for 

 the purpose ; and its due circulation over the lining membrane of 

 the shell, as well as over the outer surfaces of the intestine and 

 other viscera, is provided for by ciliary movements visible in all 

 those situations, and likewise upon the vascular laminae connected 

 with the origins of the feet.* 



Nevertheless, besides this diffused respiration, Delle Chiaje re- 

 gards a series of pinnated tentacula in the neighbourhood of the 

 mouth as being in some degree capable of performing the office of 

 branchiae. These organs, which are protruded through a row of dis- 

 tinct orifices placed around the oral aperture of the shell, are emi- 

 nently vascular ; and as they present a large surface to the action of 

 the water, and receive numerous vessels from the circular vessel 

 which surrounds the mouth, they may no doubt very well contri- 

 bute to the complete exposure of the blood to the influence of the 

 surrounding medium. 



(211.) Little is known concerning the nervous system of the 

 Echini : a few delicate filaments have been observed in the neigh- 

 bourhood of the oesophagus, apparently of a nervous character, 

 which renders it probable that a nervous ring is placed in that vici- 

 nity, resembling that already described in Asterias ; its presence, 



* Dr. Sharpey, loc. cit. 



