BLOOD SYSTEM. 



405 



water is stirred about by ciliary currents and the blood 

 in all the viscera aerated. 2. The second method is by 



771 "WP^f 



Fig. 285. — Section through one arm of a starfish : si, stomach ; m, mouth ; 

 cp, caecal pouches ; amp, ampullae ; tf % tube feet ; cv, circular vessel or 

 heart from which go the blood vessels ; a a, openings into the perivis- 

 ceral cavity. 



the vesicles of the ambulacra. On the underside of each 

 of the five arms there is a longitudinal space perforated 

 with rows of holes (ambulacra), through which project 

 hollow tentacles, which are used for walking (tube feet). 

 Each hollow tentacle is connected with a vesicle within 

 (ambulacral vesicle, or ampul- 

 la), so that corresponding with 

 the rows of ambulacral tenta- 

 cles on the outside there are 

 rows of ambulacral vesicles 

 within (Fig. 285 and Fig. 286). 

 These vesicles are connected 

 with the aquiferous system, 

 characteristic of echinoderms, 

 in such wise that they can be 

 filled with water and again par- 

 tially emptied and the water 

 changed. 



Circulation and Aeration. — The heart in these is a pul- 

 sating organ, extending from a vascular ring under the 

 dorsal skin above to another vascular ring surrounding 



Fig. 286. — Transverse section of 

 an arm of a starfish : amp, 

 ampulla ; tf, tube feet. (Aft- 

 er Parker.) 



