Respiration in Invertebrate Animals. 255 



regular points, with a view to approximate as closely as possible 

 the chylaqueous fluid to the external medium, no open perforation 

 anywhere exists in the tentacular or integumentary processes. 

 The surrounding fluid cannot therefore penetrate directly from 

 without into the peritoneal cavity*. It is introduced through 

 the mouth and the digestive system. 



In the Ophiurida and Ophiocomidae, the visceral cavity is filled 

 with a fluid, which, though not so highly organized as that of the 

 preceding genera, is undoubtedly the same system : it is not so 

 milky in appearance ; it approaches more to that of pure salt- 

 water. Its floating corpuscles are far less abundant, and more 

 indefinitely formed ; its oscillations are ceaseless under the move- 

 ments of the arms and action of cilia. At the flexures of the 

 articulated pieces of the arms, soft, membranous, hollow processes, 

 opening into the peritoneal cavity, protrude. 



They are designed to aerate the fluid contents of the visceral 

 chamber : unlike the membranous integumentary projections of 

 the Asteridse, they are neither ciUated within nor without. They 

 may be seen perfectly in the smaller species, as transparent 

 objects. If any vestige of a blood-proper system of vessels oc- 

 cupied the substance of their parietes, it could not, thus examined, 

 escape detection ; none such exists. Wherefore then are these 

 specific organs provided, if not to arterialize the great system of 

 fluid which penetrates into their interior ? The answer cannot 

 be withheld ; it is to aerate the chylaqueous fluid exclusively f. 

 The chylaqueous system of the Echinidse (fig. 9), comprehending 

 a considerable mass of fluid filling the cavity of the spherical shell 

 {d), has never yet been recognised by the anatomist as a vital 

 organic system. Thegreat authorities, Agassiz especially, formerly 

 quoted, state that sea water streams into the visceral cavity 

 through perforations in the membranous processes (fig. 9, /, /) 

 of the shell, especially in those under the name of branchice, which 

 are distributed in groups around the circumference of the oral 

 membranous disc. The latter are not connected with the suc- 

 torial or water-vascular system ; they are distended by injections 

 thrown into the open chamber of the shell. 



They are protruded only by the force of the fluid driven into 

 their interior. They collapse by contractility of their parietal 



* The author would again refer the reader to his recent memoir in the 

 Phil. Trans. (1852), for a full and complete statement of the anatomical 

 and experimental evidence, by which ai"e substantiated the general proposi- 

 tions enounced in the text. 



t It will be afterwards shown, that comparative anatomy has done ab- 

 solutely nothing towards the demonstration of the blood system of these 

 Echinoderms. A cucular vessel is stated by Miiller only to surround the 

 mouth. 



