ECHINODERMATA 229 
stone canal. The stone canal is lined by a ciliated epithelium, 
surrounded by calcified connective tissue, a ridge projects into 
its interior, and the free edge of the ridge may bifurcate, each 
half then folding back upon itself. The circumoral ring bears 
nine glandular bodies, composed of branching tubules lined 
with cubical cells, and opening into the ring. ‘These bodies 
are known as Tiedemann’s bodies. The stone canal opens in 
the position where the tenth of these bodies should be. It is 
possible that the corpuscles which float in the fluid of the 
water-vascular system are formed in these bodies. 
The radial vessels which pass along the arms lie ventral 
to the ambulacral plates, between them and a transverse muscle 
which runs between each pair (Fig. 131). Opposite each 
tube-foot the radial vessel gives off a transverse branch. Each 
branch passes between the ambulacral ossicles, and opens into 
a vesicular expansion, the ampulla, situated in the coelom. 
From this another vessel passes to the tube-foot. The con- 
traction of the ampulla forces fluid into the tube-foot, and so 
extends it. At the tip of the arm the radial tube ends in an 
unpaired terminal tentacle, at the base of which is a thicken- 
ing beset with eyes. The tentacle has a very well-developed 
nervous layer. 
The blood system described by German authors is founded 
on misinterpretation. They describe a radial vessel, an oral 
ring, and an aboral ring, and a connecting heart lying inside 
the corresponding organs described above. The radial and oral 
vessels are nothing but the thickened septa of the true vessels, 
the heart is a solid glandular organ, and the aboral vessel is 
the genital rhachis, partly degenerate. The rhachis is in 
connection with the so-called heart. 
The nervous system is diffused all over the body, but 
better developed in some parts than in others. The epidermis 
contains numerous sense cells, prolonged at their bases into 
nerve fibrils; these are not very abundant on the dorsal 
surface, but along the ridge which lies between the tube-feet, 
and in a ring which surrounds the mouth, both sense cells and 
nerve fibres exist in great quantities. The triangular ridges 
which occupy the ventral surface of the arms unite in a ring 
round the mouth, and constitute the central nervous system 
