CHYLAQTJEOUS FLUID. SAND-CANAL. 181 



membranous appendages, lined within and without by a ciliated epithe- 

 lium. A few proper blood-vessels reach their bases from the circular 

 vessel ; but no trace whatever of a vascular plexus in the structure of 

 these parts can be detected. The inference is that the tentacles are 

 designed for the oxygenization of the chylaqueous fluid. To the genus 

 Holothuria the same observations are strictly applicable ; but although 

 attenuated at regular points, with a view to approximate as closely as 

 possible the chylaqueous fluid to the external medium, no open perfora- 

 tion anywhere exists in the tentacular or tegumentary processes. The 

 surrounding fluid therefore cannot penetrate directly from without into 

 the peritoneal cavity ; it is introduced through the mouth and digestive 

 system. 



(476.) Before quitting this part of our subject, we must briefly men- 

 tion 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 fig. 90, enclosed in the same sheath as the 

 dilated vessel, /, upon the right side of which it is placed ; it communi- 

 cates by one extremity with an isolated calcareous mass, of a rounded 

 figure, called the madreporic plate, seen upon the exterior of the dorsal 

 surface of the Star-fish, while by its opposite extremity it opens into 

 the circular sinus that surrounds the mouth. The tube itself Dr. Sharpey 

 describes* as being about the thickness of a surgeon's probe, and com- 

 posed 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 them- 

 selves 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 disk, appear- 

 ing gradually to become 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 disk, 

 by means of the calcareous tube, which will thus serve as a sort of filter 

 to exclude impurities. 



(477.) Apparently with a view to ensure a continual circulation of 

 aerated fluids through all parts of the system, the entire surface of the 

 membrane that lines the shell, as well as that which forms the external 

 tunic of the digestive organs, has been found to be covered with multi- 

 tudes of minute cilia, destined by their ceaseless action to produce 

 currents passing over the vascular membranes, and thus to keep up a 

 * Cyclopaedia of Anatomy and Physiology, art. " ECHINODERMATA." 



