Respiration in Invertebrate Animals. 257 



by which the \asceral cavity (fig. 8, h) is filled is considerable iu 

 volume. The whole integumentary structures are more readily 

 subjected to demonstration. Everything is favourable to a final 

 conclusion of" the controversy which has long divided anatomists 

 as to the real signification of the fluid contained in the visceral 

 cavity : it can be placed, in several modes, beyond doubt, that 

 no open perforations exist in any part of the integumentary 

 parietes of Asterias. The membranous processes (fig. S,f,f) 

 openly communicating with the visceral cavity are so remarkably 

 elastic and protrusile, that, by means of coloured size forced 

 carefully into the cavit)', they distended to a great distance 

 above the plane of the external surface. They are ccecal at their 

 distal extremities. 



This injection escapes externally only by rupture : this simple 

 expedient proves the cjecal character of these parts ; they are not 

 consequently designed to admit sea water into the interior of the 

 body. It is perfectly easy to repeat and confirm the first obser- 

 vation of Dr. Sharpey, that the corpuscles of the visceral fluid 

 advance to the distal end of these processes, and then return 

 under the impulse of ciliary agency*. Although an injection so 

 thick as size will not escape through these membranous pro- 

 cesses, a thinner fluid, such as coloured water, will slowly ooze 

 through ; it is not therefore improbable that an interchange of 

 the fluids, which their attenuated parietes only divide, may to 

 some extent occur through endosmose. This fact, however, can- 

 not shake the stability of the conclusion, that anatomy does not 

 furnish any grounds for the belief that the fluid contained in the 

 peritoneal caN^ty is derived directly from without. The micro- 

 scope renders it certain that the hollow membranous processes, 

 filled by the fluid of the \isceral cavity in Asterias, bear in the 

 solid substance of their parietes no trace of true blood-vessels ; 

 they are lined within and without by \'ibratile epithelium, and 

 composed only of interlacing elastic fibres. What conceivable 

 office can such organs execute, if not that of exposing the chyl- 

 aqueous fluid to the renovating influence of the sun-ounding 

 medium ? In Asterias this fluid approaches " simple sea water " 

 closely in physical pi'operties. It is, however, in reality a dilute 

 albuminous, opalescent solution. It is charged scantily with 



* Miiller confirms the conclusions stated in the text — " Die respirato- 

 rischen Rohrchen auf dera Riicken der Asterien, welche mit der Bauch- 

 hohle communiciren soUen zufolge der Injection von Tiedemann am Ende 

 ofFen sein, iind zum Wechsel des Wassers des Leibeshohle dienen. Nach 

 Ehrenberg dagegen sind die Rohrchen am Ende geschlossen, er sowohl als 

 Sharpey sahen die Stromungen im Innern am Ende umkehren. An jun- 

 geu lelicnden Exemplaren des Aster acanthion violaceus sah ich dasselbe, 

 und es gelang mir nicht eine Oeffnung wahrzunehmen." — Anatomische 

 Studien Uber die Echinodermen. Midler's Archiv, 1850. 



Ann. &c Mag. N. Hist. Ser. 2. Vol. xii. 18 



