2-t2 



ZOOLOGY 



become five large, bony jaws — constituting together the 

 "lantern of Aristotle " (Fig. 233 j. These jaws are worked 

 by powerful muscles, and each carries a chisel-shaped tooth 



that is capable of continuous 

 growth. It would be difficult 

 to find in nature adaptations 

 more complete than the 

 transformation of twenty- 

 five incipient plates into five 

 powerful and effective jaws. 

 The food, torn up bj' the 

 jaws, passes into the diges- 

 tive tube which, instead of 

 extending straight to the 

 aboral pole, forms a large 

 loop in the ho&y. No dis- 

 tinction between stomach 

 and intestine is to be made 

 out, and there are usuallj' no 



Fig. 2.32. — The ■^■;lfor-^■as(■ular system of large glands; but in most 

 a starfish, m, niadreporite, connecting u- ■ i j-i, • i j. u 



K„„.„r,o' ( '> ^,;ti,n, ■ 1 ^ echmoics there IS a long tube 



byacanai, (m'j, with the circular canal " 



(c), from which the radial vessels (r), lying alongside the intestine 



{siph, Fig. 234), opening 

 from it near the mouth and 

 emptying into it again at the 

 middle third of its course. 



It is possible that this tube serves to supply oxygen to the 



blood vessels that go to the intestine prunarily for liquid 



food. 



Organs of Respiration. — These are very varied in the 



different classes of the echinoderms, and probably in each 



run to each arm and on which lie the 

 vesicles (np) that serve as reservoirs. 

 From the radial vessels (r) tubes (/■'), 

 are given off to the tube-feet and their 

 bladders (a). From Parker and Has- 

 well. 



