STARFISH 



197 



226. Water-vascular System. — The water-vascular system, which is 

 pecuhar to the echinoderms, is a system of canals filled with water, which 

 serves as a locomotor system (Fig. 111). The madreporite on the aboral 

 surface, when examined closely, is seen to be marked by fine radiating 

 grooves along which are numerous minute openings, making of the whole 

 plate a sieve. Through this water enters the system and is strained as 

 it does so. From the madreporite a stone canal, so called because of 

 lime in its walls, runs downward across the body ca\ity to a ring canal 

 which lies at the outer margin of 



Mac/repor/'-/-e 

 ■5 to fie caA7a/ 



c-cr/7cr/ 



Rac//a/ 



Po/i'a/7 

 ves/c/e 



■ff/n^ rar7a/ 



Ar77pc//a 



la-/-era/ 

 ccr/7a/ 



Sucker 



the perioral membrane. In the 

 stone canal just below the madre- 

 porite, as well as on the inner sur- 

 face of the madreporite itself, are 

 ciha, the movement of which forces 

 water into the canal. From the 

 ring canal a radial canal runs out 

 each ray, lying at the bottom of 

 the ambulacral groove. From this 

 radial canal arise lateral canals 

 which lead to the tubes connecting 

 the ampullae and the tube feet. 

 The ampullae are sacs lying inside 

 the wall of the ray; the tube feet 

 lie outside it and in the ambulacral 

 groove. The tube which connects 

 each ampulla with its tube foot 

 runs through an ambulacral pore 

 (Fig. 110 A), formed by the separa- 

 tion of adjacent ossicles. These 

 pores are in two rows on each side corresponding to the two rows of tube 

 feet and the lateral canals are thus alternately longer and shorter. In 

 the walls of the ampullae are circularly arranged muscle fibers which by 

 their constriction, working on the principle of a bulb syringe, lessen the 

 capacity of these ampullae. The tube feet are hollow organs cylindrical 

 in form, in many cases with a sucker at the outer end, working on the 

 principle of a vacuum cup. The lateral walls of these organs contain 

 muscle fibers. 



The manner in which locomotion is accomplished is as follows : When 

 the animal is about to move in a given direction, the ray — or rays — point- 

 ing in that direction is raised and the tube feet are elongated as water is 

 forced into them by the contraction of the ampullae. When these tube 

 feet have been extended as far as possible, which in a starfish a foot in 

 diameter may be as much as two inches, their outer ends are brought in 

 close apposition to the object upon which the starfish is resting and the 



Fig. 111. — Diagram to illustrate the 

 arrangement of parts in the water-vascular 

 system, a, tube foot and ampulla, enlarged. 



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