ECHINODERMATA. 157 



rect. Professor Sharpey agrees with Tiedemann in the opinion that 

 the vessels of the feet form a system perfectly distinct from that of 

 the blood-vessels, and even supposes that the fluid by which the 

 ambulacra! tubes become distended is neither more nor less than 

 pure sea-water. 



(196.) Before quitting this part of our subject, we must briefly 

 mention a singular organ, apparently intimately connected with the 

 circular vessel around the mouth, and called by Tiedemann the sand 

 canal. This organ is represented in j#g. 67, enclosed in the same 

 sheath as the dilated vessel (/), upon the right side of which it is 

 placed ; it appears to communicate by one extremity with an isolated 

 calcareous mass of a rounded figure, seen upon the exterior of the 

 dorsal surface of the star-fish, while by its opposite extremity it 

 opens apparently into the circular sinus which surrounds the mouth. 

 The tube itself Dr. Sharpey describes* as being about the thickness 

 of a surgeon's probe, and composed of rings of calcareous substance 

 connected by a membrane, so that viewed externally, it is not unlike 

 the windpipe of a small animal. On cutting it across, it is found to 

 contain two convoluted laminae of the same nature as its calcareous 

 parietes, which are rolled upon themselves in a longitudinal direction 

 in the same manner as the inferior turbinated bones of an ox. The 

 convoluted arrangement becomes more complete towards the upper 

 end of the tube, where the internal laminae, as well as the external 

 articulated portion, join the dorsal disc, appearing gradually to be- 

 come continuous with its substance. The use of this curious organ 

 is quite unknown, although a variety of conjectures have been 

 hazarded upon the subject. The most probable appears to be that 

 of Dr. Sharpey, who suggests that, should the fluid which distends 

 the feet and the vessels connected with them be indeed sea- water, 

 it may be introduced and perhaps again discharged through the 

 pores of the disc by means of the calcareous tube, which will thus 

 serve as a sort of filter to exclude impurities. 



(197.) The Asterias possesses no organs specially appropriated 

 to respiration ; but the sea-water, being freely admitted into the 

 general cavity of the body through a set of minute membranous 

 tubes seen upon the exterior of the animal, bathes all the viscera, 

 and consequently ensures a complete exposure of the circulating 

 fluids to the influence of oxygen, the whole peritoneal surface per- 

 forming the office of a respiratory apparatus. The mechanism by 



* Cyclopaedia of Anatomy and Physiology j art. Echinodermata. 



