100 ZOOLOGY. 



it is conveyed to each water-sac or ampulla (Fig. 62, am). 

 These pear- shaped water-sacs, when contracted, are supposed 

 to press the water into the long slender suckers or ambulacral 

 feet, which are distended, elongated, and by a sucker-like ar- 

 rangement at the end of the prehensile foot act in conjunc- 

 tion with the others to warp or pull the star-fish along. 

 Besides locomotion the ambulacral feet serve for respiration 

 and perception (Simroth). Hoffman shows that the feet 

 of the sea-urchins can be projected or thrust out without 

 the aid of the ampullae. 



It will thus be seen that the water-vascular system in the 

 star-fish is in its functions partly respiratory and partly 

 locomotive, while it is in connection with the vascular sys- 

 tem, and thus partly aids in circulating the blood and 

 chyle. 



Of the true vascular or blood system the student can ordi- 

 narily only discover one portion, the so-called " heart " or 

 " pulsating vessel," which we may call the haemal canal (Fig. 

 62 7^), and which runs parallel to the stone-canal from the 

 madreporic body to near the ring-canal.* It is nearly as 

 large as the stone-canal, slightly sinuous, muscular, and with 

 the latter is surrounded by a loose investing membrane like 

 a pericardium. Some observers deny the existence of a vas- 

 cular (sometimes called " pseudohaemal ") system, but it has 

 been recently studied by Hoffman and subsequently by Teu- 

 scher, who maintains that in all Echinoderms there are two 

 systems of blood-vessels, which belong, one to the viscera and 

 the other to the nervous system, forming an oral or nervous 

 ring and an anal ring. The two rings are in direct com- 

 munication in the star-fishes, Ophiurans and sea-urchins, 

 but not in the Holothurians. The radial nerves are ac- 

 companied by a vessel which subdivides and distributes 

 branches to the ambulacral feet in star-fishes, Echini, and 

 Holothurians. Teuscher considers that the " heart " found 

 in the star-fishes and Echini connecting the oesophageal (or 

 nerve-ring) and anal ring, is neither a gland nor a pulsating 

 vessel, as different authors have supposed, but perhaps only 



* Simroth states that in Ophiurans (Ophiactis) the stone-canal opens 

 in common with the " heart" into the madreporic plate. 



