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 Ijy 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 pouclies 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 

 tlie 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 strvic- 

 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 tlie 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 



