5i8 



ECHINODERMATA ECHINOIDEA 



(Fig. 232, oss), a circumstance which enabled Johannes Miiller to 

 recognise the Ecliinoid larva when the form of the adult was 

 as yet unrecognisable. Below the edge of the disc there is a 

 well-marked nerve-ring, from which two bundles of nerve-fibres 

 go to the disc itself, in tlie edge of which there is an abundance 

 of sense-cells. 



The buccal tube-feet (Fig. 229, 4) are much sliorter than the 

 rest, and are provided with oval discs which are highly sensory. 

 These feet are not used for seizing, but for tasting food ; when a 



amp 



Fig. 232. — Diagrammatic transverse section of tlie radius of an Echinoid. amb.oss, 

 Ambulacral ossicle ; amp, ampulla of the tube-foot ; ej), epineural canal ; muse, 

 muscles attaching spine to its boss ; nerv, nervous ring in base of spine ; n.r, radial 

 nerve-cord ; oss, ossicle in sucker of tube-foot ; ped, tridactyle pedicellaria ; perih, 

 radial perihaemal canal ; pod, tube-foot ; wv.r, radial water-vascular canal. 



piece of food is placed near them they are thrown into the most 

 violent agitation. 



The nervous system has the same form as in an Asteroid, 

 viz. that of a ring surrounding the mouth and giving off radial 

 nerve-cords (Fig. 232, n.r), one of which accompanies each water- 

 vascular canal to the terminal tentacle, where it forms a nervous 

 cushion in which pigmented cells are embedded. 



A large band-like nerve is given off from the radial nerve- 

 cord to each tube-foot. This pedal nerve, as it is called, contains 

 bipolar neurons, and is really an extension of the nerve-cord 

 itself Beneath the sucker it branches out to form a sensory 

 ring. From the base of the pedal nerve, branches are given off 



