444 ECHINODERMATA — ASTEROIDEA chap. 



that the loss thus suffered is made up by the entry of fresh fluid 

 from the radial canal. The radial canal in turn draws from the 

 ring-canal, and this last is supplied by the stone-canal, the 

 cilia of which keep up a constant inward current. 



In the fluid contained in the water-vascular system, as in the 

 coelomic fluid, there are amoebocytes floating about. These are 

 produced in short pouches of the ring-canal, nine in number, 

 which are called after their discoverer " Tiedemann's bodies " 

 (Fig. 190, T). From the cells lining these the amoebocytes are 

 budded off 



The nervous system of the Starfish is in a very interesting 

 condition. The essential characteristic of all nervous systems is 

 the presence of the " neuron," a cell primitively belonging to an 

 epithelium but which generally has sunk below the level of the 

 others and lies amongst their bases. This type of cell possesses 

 a round body produced in one direction into a long straight 

 process, the " axon," whilst in the other it may have several 

 root-like processes, or " dendrites," which may spring from a 

 common stem, in which case the neuron is said to be " bipolar." 

 The axon is often distinguished as a " nerve-fibre " from the round 

 body which is termed the " nerve-cell." This is due to the fact 

 that for a long time it was not recognised that these two struc- 

 tures are parts of a whole. 



Now at the base of the ectoderm all over the body of the 

 Starfish there is to be found a very fine tangle of fibrils ; these 

 are to be found partly in connexion with small bipolar neurons 

 lying amongst them and partly with isolated sense-cells scattered 

 amongst the ordinary ectoderm cells. This nervous layer is, 

 however, very much thickened in certain places, so as to cause 

 the ectoderm to project as a ridge. One such ridge is found at 

 the summit of each ambulacral groove running along the whole 

 under surface of the arm and terminating in a cushion at 

 the base of the median tentacle of the water-vascular system. 

 This ridge is called the radial nerve-cord. The five radial 

 nerve -cords are united by a circular cord, the nerve -ring, 

 which appears as a thickening on the peristome surrounding 

 the mouth. 



The sense-organs of the Starfish are chiefly the discs of the 

 tube-feet. Eound the edges of these there is a special aggrega- 

 tion of sense-cells ; elsewhere, as in the skin of the back, only 



