182 ECHINODERMATA. 



perpetual supply of oxygenated water to every part*. But it is not 

 only on the peritoneal surfaces that the existence of cilia has been 

 detected ; they are found to be extensively distributed over the external 

 surface of 'the body, within the cavities of the tubular feet, and even 

 over the whole internal lining of the stomach and cseca. 



(478.) " In Asterias rubens," says Dr. Williams, " it can be distinctly 

 demonstrated that no open perforations exist in any part of the integu- 

 mentary parietes. The membranous processes communicating with the 

 visceral cavity can be proved, by injection, to be csecal at their distal 

 extremities. . It is easy to repeat and confirm the observation 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. Nevertheless, although an injection so thick as size will not 

 escape through these membranous processes, a thinner fluid, such as 

 coloured water, will slowly ooze through ; it is not, therefore, impro- 

 bable that an interchange of the fluids may to some extent occur through 

 endosmose. The microscope renders it certain that the hollow mem- 

 branous processes filled by the fluid of the visceral cavity, in Asterias, 

 bear in their parietes no trace of true blood-vessels : they are lined 

 within and without by vibratile epithelium, and composed only of in- 

 terlacing elastic fibres ; and consequently their only office seems to be 

 that of exposing the chylaqueous fluid to the renovating influence of 

 the surrounding medium. In Asterias, this fluid approaches simple sea- 

 water closely in its physical properties. It is, however, in reality a 

 dilute, albuminous, opalescent solution. It is charged scantily with im- 

 perfectly formed corpuscles, always the same in the same species ; and 

 the proposition may now be confidently affirmed, that in the Echinoder- 

 mata the chylaqueous fluid (i. e. the contents of the visceral cavity) is 

 itself first aerated, and that by means of a machinery of soft parts it 

 then aerates the blood proper." 



(479.) The organs belonging to the reproductive system in the Aste- 

 ridce exhibit the greatest possible simplicity of structure. The ovaria 

 (fig. 88, 1, /) are slender ca3ca, arranged in bunches around the oeso- 

 phagus, two distinct groups being lodged at the origin of each ray. In 

 Asterias aurantiaca (fig. 90), the excretory ducts are not easily seen ; 

 but in the Twelve-rayed Star-fish, especially if examined when these 

 organs are in a gravid state, each ovary may be observed to communi- 

 cate externally by a wide aperture that perforates the osseous circle 

 encompassing the mouth. 



(480.) The generative organs of the male individuals exactly resemble 

 those of the female, and are only distinguishable by the presence of 

 spermatozoa in their interior. The process of reproduction f usually 



* See the article " CILIA," by Dr. Sharpey, in the ' Cyclopaedia of Anatomy and 

 Physiology.' 



f M^moire siir le Bevel oppement des Astries, par M. Sars, Ann. des So. Nat. 1844. 



