XIV] ASTERIAS 335 



The nervous system of the star-fish is one ot the most interesting 

 Nervous features in its anatomy. The ectoderm consists of long 

 system. delicate cells bearing flagella and interspersed with 



goblet-cells similar in appearance to those lining the stomach. 

 The slime which these cells manufacture covers the surface of the 

 animal and no doubt protects it from the attacks of bacteria and 

 microscopic algae. But the chief point of interest is that at the 

 bases of the long delicate cells there is an indescribably fine tangle 

 of delicate nerve-fibres which are doubtless outgrowths of some of 

 the cells. Here and there a nucleus is seen amongst them which 

 belongs to a neuron that is, an ectoderm cell which has lost its 

 connection with the rest and has become pushed down into the 

 fibrillar layer. The ectoderm all over the body is therefore under- 

 lain by a nervous sheath and is very sensitive, but there are certain 

 places where the nervous sheath becomes very much thickened and 

 it is these areas which constitute the true sense-organs and the 

 central nervous system. 



Isolated sense-cells, that is, cells having a stiff protruding hair, 

 are scattered all over the surface ; but the only spot where they are 

 collected in groups so as to form the sense-organs is on the tips of 

 the tube-feet. The tube-feet are then practically the only sense- 

 organs, and since the radial water-tube ends at the tip of an arm in 

 an unpaired tube-foot devoid of a sucker, we might regard the whole 

 radial tube as a huge, branched, sensitive tentacle. There is the more 

 justification for doing this when it is found that the radial tube with 

 its freely projecting tip is in the young star-fish quite independent 

 of the outgrowth of the body called the arm, and only secondarily 

 becomes applied to it. At the base of the end-tentacle there is a 

 thick cushion of nervous matter in which are excavated a number 

 of ectodermal pits lined by cells containing orange pigment. 

 These pits are organs of vision: and it has been experimentally 

 shown that a star-fish deprived of these organs is insensible to 

 light. 



The central nervous system consists of five thick bands of 

 nervous tissue situated one above each ambulacral groove under- 

 neath the radial water-tube (Fig. 153). They are termed the radial 

 nerve-cords and are joined together by a circular band of a similar 

 nature, called the nerve-ring, lying under the water- vascular ring. 

 Intervening between the radial nerve-cord and the radial water-tube 

 there are two canals lined by flattened cells and separated from one 

 another by an imperfect septum (14, Fig. 153). They are called 



