IX 



PHYLUM ECHINODERMATA 



399 



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

 suing a corresponding course, runs a radial anibulacral 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 ampullae (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 (pal. 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, 



perUi 



amp. 



ois 



FIG. 326. Diagrammatic transverse section of the ambulacral zone of an Echinoid. amb. os. 

 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 ; 

 oss. ossicle in the sucker of the tube-foot ; ped. pedicellaria ; perih. radial perihsemal canal ; 

 pod. tube-foot ; icv. r. radial ainbulacral 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 caeca 

 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 ccelomic fluid. 



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

 there are numerous corpuscles. Of these there are two kinds 

 amoeboid corpuscles (amcebocytes) with long pseudopodia, and 

 vibratile corpuscles, which closely resemble sperms, having a rounded 



