416 COMPARATIVE ANATOMY CHAP. 



VI. The Water Vascular System. 

 (System of the Ambulacral Vessels : Hydroccel.) 



This is a system of canals filled with fluid, the arrangements of 

 which may be generally described as follows. 



An outer aperture, the madreporite, leads first into a vesicular 

 section of the ccelom, the madreporitie ampulla. This again is con- 

 nected by means of a stone canal (so called because that portion of its 

 wall which consists of connective tissue is often calcified) with a ring" 

 canal which surrounds the oesophagus. Into the madreporitie ampulla 

 there opens, further, the axial sinus of the body cavity, which follows 

 the stone canal in its course, and surrounds a lymphatic gland, the 

 axial organ. 



The water vascular ring 1 may carry various accessory structures, 

 whose principal function seems to be that of lymphatic glands, and 

 which are known as Polian vesicles, Tiedemann's bodies, etc. 



From the ring canal there run out into the radii of the body, 

 either in the body wall or in close contact with it, as many radial 

 canals as there are radii (usually therefore five). The radial canals 

 send off, on each side, tube-feet canals, which run into outer append- 

 ages of the body wall, ending blindly at their tips. These extensible 

 appendages are usually present in great numbers, and serve either as 

 tube-feet for locomotion (Holothurioidea, some Echinoidea, Asteroidea), 

 and are then provided with a terminal sucker, or as tentacles, ten- 

 tacular gills, etc., for tactile purposes, for respiration, and for conduct- 

 ing food (some Echinoidea, Ophiuroidea, Crinoidea). In connection with 

 the tube-feet canals, tube-feet ampullae are very often found (Holo- 

 thurioidea, Echinoidea, Asteroidea) ; these are accessory contractile 

 vesicles, which serve for the swelling of the tube -feet. Special 

 valves are so arranged as to prevent the flowing back of the water 

 vascular fluid into the radial canals (Fig. 352, p. 409). 



The chief departures from this general description met with in 

 the five classes of Echinoderms, affect the madreporite, the madre- 

 poritie ampulla, and the stone canal. These will be described in 

 detail later on. 



Structure of the wall of the water vessels. Lining the lumen of the vessels, 

 there, is generally found, first of all, a ciliated epithelium. This is followed, in 

 most parts (always in the ambulacral appendages), by a longitudinal muscle layer. 

 Outside this latter lies a layer of connective tissue, and, outermost of all, there is 

 almost always an external ciliated epithelium. On the ambulacral appendages (the 

 tube-feet and tentacles) this last is nothing more than the external body epithelium. 

 But in those parts of the water vascular system winch project into or lie in the 

 body cavity, it is the endothelium of the ccelom. This outer epithelium of the 

 water vascular system is rarely altogether wanting ; it is, however, absent in such 

 parts of the system as run embedded in the body wall. A circular musculature is 

 seldom found ; it only occurs locally. 



