BODY FLUIDS AND CIRCULATION 97 



Hydraulic mechanisms are well developed in echinoderms, where they 

 are involved in locomotory, feeding and burrowing activities. In these 

 animals the coelom comprises (1) the perivisceral cavity, (2) a perihaemal 

 system consisting of a perihaemal radial vessel in each radius, plus a ring 

 vessel about the mouth, and (3) a water vascular system. The latter originates 

 in an external opening, the madreporite, which is connected via an axial 

 sinus and stone canal with a circumoral water vascular ring. The ring 

 bears several Polian vesicles and from it a radial water vessel enters each 

 radius and gives off lateral vessels to the tube feet. In crinoids and holo- 

 thurians the madreporite opens into the perivisceral cavity. In addition 

 there is a lacunar vascular system consisting of a ring of lacunar tissue 

 around the mouth, from which a longitudinal strand penetrates into each 

 radial perihaemal canal. In holothurians and echinoids dorsal and ventral 

 vessels from the oral lacunar ring pass along the alimentary canal, on 

 which they form plexuses. In some sea cucumbers the dorsal intestinal 

 vessel gives rise to an intricate rete mirabile on the mesentery, and this rich 

 network also becomes associated with the respiratory trees (29). 



In those echinoderms in which the tube feet are used for walking, the 

 radial water vessels bear contractile ampullae associated with the tube feet. 

 These ampullae are able to drive water into the podia, causing the latter 

 to protract (p. 429). A valve at the junction of the radial vessel and the 

 lateral canal leading into each ampulla confines fluid to the ampulla- 

 podial system during motor activity. 



In holothurians the anterior podia are modified into tentacles, which 

 are supplied by radial ducts of the ambulacral system. This originates in 

 an oral ring which bears Polian vesicles, one or more sinuses leading into 

 internal madreporites, and a set of five radial ducts. The latter connect 

 with the tentacles and the tube feet, and the Polian vesicles form a reservoir 

 of fluid for the entire podial system. The tentacular ducts leading off the 

 radial vessels are guarded by valves, and usually possess tentacular ampul- 

 lae which are employed in extruding the tentacles. Pressures vary indepen- 

 dently in the Polian complex and tentacular ducts. In Thyone pressure is 

 usually higher in the tentacular duct than in the Polian complex (10-25 cm 

 as against 3-5-5 cm H 2 0). In a relaxed animal pressure in the body cavity 

 is low (0-2-5 cm H 2 0), but rises during activity to 37 cm H 2 0. Similar 

 values are given for Caudina. 



Thyone and Caudina are burrowing animals, and it is interesting to 

 compare them with Holothuria which inhabits rock crevices. In H. grisea 

 rhythmical cloacal contractions drive water into the cloaca and respiratory 

 trees; after about ten contractions the cloaca opens and the accumulated 

 water is driven out by contraction of the body wall. This expulsion 

 generally results in a fall of about 5 mm in coelomic pressure (Fig. 3.4). 

 Average coelomic pressure varies from 7 to 21 mm H 2 0, and during 

 normal body movements the coelomic pressure approaches 30 mm H 2 0. 

 Strong mechanical stimulation producing contraction of the body wall 

 brings about a pressure rise of 4-5-16 cm H 2 (Fig. 3.5). A comparison of 



M.A.- 



