IX 



PHYLUM ECHINODERMATA 



301 



each radial nerve is connected with the so-called eye (oc), borne 

 by the corresponding ocular plate. These parts correspond to 

 the epidermal nervous system of the Starfish, which, owing to 

 the ambulacral grooves having become closed in to form narrow 

 canals the epineural canals (Fig. 331, ep.), covered over by the 

 plates of the corona is here more deeply situated ; the deep and 

 ccelomic systems are only feebly developed. 



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

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

 330 and 331). 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 ampullce (amp), by two canals, one 

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



pei-ih 



amp 



FIG. 331. Diagrammatic transverse section of the ambulacral zone of an Echinoid. a nib. oss. 

 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 ; wv. r. radial ambulacral vessel. (After MacBride.) 



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, 

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

 ring-canal. 



The enteric canal (Fig. 332, 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 diverticulum, the siphon (siph), which opens 



