158 ECHINODERMATA. 



which the surrounding element is thus drawn into the body, and 

 the process by which its expulsion is effected, are not accurately 

 known ; nevertheless, apparently with a view to ensure a continual 

 circulation of aerated water through all parts of the system, the 

 entire surface of the membrane which lines the shell, as well as that 

 which forms the external tunic of the digestive organs, has been 

 found to be covered with multitudes of minute cilia, destined by 

 their ceaseless action to produce currents passing over the vascular 

 membranes, and thus to ensure a 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 caeca. 



This amazing apparatus of vibratile cilia must necessarily serve 

 some important purpose in the economy of these creatures ; and 

 Professor Sharpey, to whose observations upon ciliary motion phy- 

 siology is deeply indebted, regards them as being most probably 

 subservient to respiration. 



(198.) The organs belonging to the reproductive system in the 

 Asterida exhibit the greatest possible simplicity of structure : there 

 is no distinction of sex, neither have any parts been discovered in 

 connection with the ovigerous organs, which can be regarded as 

 ministering an accessory secretion. The ovaria (fig. 67, /, /) are 

 slender cseca arranged in bunches around the oasophagus, two -dis- 

 tinct groups being lodged at the origin of each ray. In Asterias 

 aranciaca (fig. 67), 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 com- 

 municate externally by a wide aperture, which perforates the osseous 

 circle encompassing the mouth. (Fig. 65, f.) 



(199.) In order to complete the history of the Asterida, we have 

 yet to mention the nervous apparatus with which they are furnished. 

 This consists of a simple circular cord, which runs around the 

 mouth of the animal ; from this ring, three delicate filaments are 

 given off opposite to each ray, one of which, according to Tiede- 

 mann, runs along the centre of the ambulacral groove upon the 

 under surface of the body, and gives minute twigs to the locomo- 

 tive suckers placed on each side of its course ; the other two fila- 



* See the article Cilia by Dr. Sharpey, in the Cyclopaedia of Anatomy and Physiology. 



