524 ECHINODERMATA ECHINOIDEA chap. 



cut through, the tube-feet in the various radii are no longer 

 co-ordinated with one another. 



Besides the tips of the tube-feet the Urchin possesses another 

 kind of sense-organ, the sphaeridia (Fig. 233). These are minute 

 glassy spheres of calcareous matter attached by connective tissue 

 to equally minute bosses on the plates of the ambulacra, generally 

 near the middle line. They are in fact diminutive spines, and 

 like the latter are covered with a thick layer of ectoderm, beneath 

 which is a particularly well-developed cushion of nerve-fibrils. 

 Only the layer of muscles which connects a normal spine with 

 its boss is wanting. Although definite experimental proof is 

 lacking, the whole structure of the sphaeridia shows that they 

 belong to the category of "balancing organs." As the animal 

 sways from side to side climbing, over uneven ground, the heavier 

 head of the sphaeridia will incline more to one side or to another, 

 and thus exercise a strain on different parts of the sheath, and 

 in this way the animal learns its position with regard to the 

 vertical. 



Intervening between the radial nerve -cord and the radial 

 vessel is a single radial perihaemal canal (Fig. 232, perih), 

 representing the two parallel canals found in the same position 

 in the Asteroid. The five perihaemal canals lead downwards to 

 a space called the lantern-coelom, surrounding the oesophagus. 1 

 Since the skeleton of the corona is composed of plates immovably 

 connected together, muscles corresponding to the ambulacral 

 muscles of the Asteroids would be useless, and so the wall of the 

 perihaemal canal remains thin and the side of it turned towards 

 the general coelom develops no muscles, and that turned towards 

 the nerve-cord no nerve-cells. Where, however, the radial nerve 

 enters the nerve-ring, and on the ring itself, an inner layer of 

 nerve-cells is developed from the lantern-coelom which represents 

 the lower or oral portions of the radial perihaemal canals. These 

 cells control the muscles moving the teeth. These canals are 

 originally parts of the lantern-coelom, but in the adult they become 

 closed off from it. In the outer wall of this space are developed 

 the calcareous rods forming Aristotle's lantern. These are first : 

 five teeth (Fig. 234, li), chisel-shaped ossicles of peculiarly hard and 

 close-set calcareous matter, the upper ends (l) pushing out pro- 



1 In the aberrant genus Asthenoso?na, where there are internal radial muscles, 

 there is also an internal series of nerve-cells on the radial cord. 



