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 



