PHYLUM ECHINODERMATA 



399 



Ambulacral System. — Internal to each radial nerve, and pur- 

 suing a corresponding course, runs a radial ambulacral vessel (Figs. 

 325 and 326). From this are given off on each side a series of short 

 branches to the tube-feet, with each of which is connected one of 

 a series of compressed sacs, the ampullm {amp), by two canals, one 

 passing through each of the two pores. At their oral extremities 

 the five radial ambulacral vessels unite with a ring-vessel 

 surrounding the oesophagus. Appended to the ring-vessel are 

 five Polian vesicles (pol. ves.) in the form of small mammillated 

 bodies. A madreporic canal (mad. can.), corresponding to that of 

 the Starfish, but with soft membranous walls devoid of ossicles, 



Fig. 320. — Diagrramniatic transverse section of the ambulacral zone of an Ecllinoid, amb. ob«. 

 ambulacral ossicle ; amp. ampulla of a tube-foot ; ep. epineural canal ; muse, muscles 

 attaching spine to its tubercle ; nerv. nervous ring in base of spine ; n. r. radial nerve-cord ; 

 088. ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perih. radial perihffiraal canal ; 

 pod. tube-foot ; icv. r. radial ambulacral vessel. (After MacBride.) 



runs from the madreporite at the side of the periproct to the 

 ring-canal. 



The enteric canal (Fig. 327, ali) is devoid of the radial cseca 

 which it presents in the Starfish : it is a wide, soft-walled tube, 

 which winds round the interior of the corona in its passage from 

 the mouth to the anus, held in place by a band of threads, the 

 mesentery, passing out from it to the inner surface of the shell. 

 It gives off a short blind diverticulum, the siphon (siph) ; this, 

 together with the intestine itself, probably acts as an organ for the 

 respiration of the coelomic fluid. 



The coelome contains a fluid in which, as in the Starfish, 

 there are numerous corpuscles. Of these there are two kinds 

 — amcehoid corpuscles (amcehocytes) with long pseudopodia, and 

 vihratik corpuscles, which closely resemble sperms, having a rounded 



