ALIMENTARY CANAL. CHYLAQUEOUS SYSTEM. 199 



no means surprising when we consider the great difficulty of tracing 

 such delicate and extensively-distributed canals. According to Delle 

 Chiaje, the course of the nutritious fluid is as follows : A large vein 

 runs along the whole length of the intestine, from the anus to 4he 

 oesophagus, where it terminates in a vascular ring surrounding the 

 mouth, into which, as in Asterias, the contractile vesicle, which he 

 considers to be a receptacle for the nutrient fluid, and the antagonist to 

 the tubular feet, likewise opens. The intestinal vein he regards as the 

 great agent 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 arteries are, 1st, a 

 long vessel to the intestine, which runs along its whole length and 

 anastomoses freely with the branches of the intestinal vein ; 2ndly, 

 five arteries to the parts connected with the mouth ; 3rdly, five dorsal 

 arteries that 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 Asterias, supply the vascular origins of 

 the innumerable protractile feet. 



(521.) The chylaqueous system of the Echinidae, comprehending a 

 considerable mass of fluid filling the cavity of the spherical shell, has 

 been generally regarded as sea-water poured into the visceral cavity 

 through perforations in certain membranous processes of the shell, 

 which have received the name of branchial, and are distributed in groups 

 around the circumference of the oral membranous disk. The latter, 

 however, according to Dr. "Williams, are not connected with the suctorial 

 or water- vascular system, but are distended by injections thrown into 

 the open chamber of the shell, being protruded only by the force of the 

 fluid driven into their interior ; they are consequently not perforated. 



(522.) In addition to the meridional rows of suctorial feet, the shell of 

 Echinus is perforated by numerous hollow membranous processes lined 

 within and without by vibratile cilia, and penetrated exclusively by the 

 fluid of the visceral cavity ; they show no traces of blood-vessels, and 

 can only subserve a respiratory purpose on the supposition that the 

 subject of that process is the chylaqueous fluid. There is, therefore, no 

 direct evidence to show that the external element enters through open- 

 ings in the integuments into the peritoneal cavity of the Echinus. 



(523.) Nevertheless, besides this diffused respiration, Delle Chiaje 

 regards a series of pinnated tentacula in the neighbourhood of the mouth 

 as being in some degree capable of performing the office of branchia?. 

 These organs, which are protruded through a row of distinct orifices 

 placed around the oral aperture of the shell, are eminently vascular ; 



