266 PHYLUM ECRINODERMA, 
new plates around the apical disc, and also by the indi- 
vidual increase of each. In a few forms the shell retains 
some plasticity. 
The nervous system consists of a ring around the mouth, 
of radial branches running up each ambulacral area, and 
of the superficial network. Tube-feet, sphzridia, pedi- 
cellarize, and spines are all under nervous control, and each 
radial nerve ends in the “‘eye specks” of the apical ‘‘ ocular 
plates.” It is probable that all the tube-feet are sensory, 
and this is certainly the main function of ten which lie 
near the mouth. 
The alimentary canal passes through Aristotle’s lantern, 
and the intestinal portion lies in two and a half coils 
around the inside of the shell, to which it is moored by 
mesenteries. It contains fine gravel, sand, and some 
organic débris. It ends near the centre of the apical disc, 
whence the pedicellarize have been seen removing the feeces. 
The spacious body cavity is lined by ciliated epithelium, 
and contains a “perivisceral” fluid, whose corpuscles have 
a respiratory pigment (echinochrome). When the fluid of 
a perfectly fresh sea-urchin is emptied out, the contained 
corpuscles unite in plasmodia, forming composite amceboid 
clots (cf. Protomyxa, etc.). 
The madreporic plate communicates with a membranous 
stone canal (calcareous in C7daris) which runs downwards 
into a circular vessel near the upper end of the lantern. 
This gives off five inter-radial transparent “ Polian” vesicles 
and five radial vessels, which run down the sides of the 
lantern and up each ambulacral area. Each radial vessel 
gives off numerous lateral branches, which communicate 
with the internal ampulle and thence with the external 
tube-feet. When the tube-feet are made tense with fluid, 
they extend far beyond the limit of the spines, and are 
attached to the surface of the rock over which the sea- 
urchin slowly drags itself. The sucker at the tip of each 
tube-foot bears a rosette of small calcareous plates; indeed, 
there is hardly any part of an Echinoderm in which lime 
may not be deposited. Before bending upwards from 
the base of the lantern, each radial vessel gives off a branch 
to two large tentacle-like tube-feet without attaching discs. 
The five pairs lie near the mouth, and are sensitive. 
