ECHINUS. a15 
along the edges of the five alveoli, and are supposed to 
innervate their muscles. This has been described as the 
“deep oral nervous system,” in contra-distinction to the 
well-known nerve-ring and radial trunks which constitute 
the superficial oral system. 
No specialised sense organs, other than the spheridia 
and the terminal tentacles of the water-vascular system | 
appear to have been discovered in Echinus esculentus. 
The tube-feet are highly sensitive to external stimuli, and 
those of the allied species, Echinus micro-tuberculatus, 
have a tubercle-like thickening of the ectoderm close to the 
sucker disc, beneath which there is a ganglionic mass of 
nerve tissue. 
VASCULAR AND CC@LOMIC SYSTEMS. 
In addition to the radial canal of the water-vascular 
system, two other canals traverse the ambulacra, in close 
relation to the radial nerve trunks. One of these is the 
epineural canal (Pl. II1., fig. 28; Pl. IV., figs. 29, 30, and 
33, r.ep.c.). It runs along the peripheral face of the nerve 
trunk, between it and the test. At the apex of the latter 
it ends blindly, in close proximity to the terminal tentacle 
of the water-vascular canal (Pl. IV., fig. 30, 7.ep.c.), and, 
at the opposite extremity, it thins out gradually as the 
mouth is approached, and does not open into the circular 
epineural canal (Pl. IIL., fig. 28, c.ep.c.) which surrounds 
the pharynx. In the Ophiuroidea the epineural canals 
have been found to arise by the growth of two lateral folds 
of the integument over the radial nerve trunks. The folds 
meet and unite and so form the canal-lhke space. It 1s 
probable that further research into the ontogeny of the 
Echinoidea will prove that their radial canals arise in a 
similar manner. 
