ANATOMY AND PHrSIOLOGY OF FCRINODEHMS 



U5 



The nervous system is mainly on the surface of the skin. 

 There is a ring of nervous tissue (Fig. 233, 7ierv. r) surround- 

 ing the mouth and sending a branch along each radius (rod. ne.). 

 Each branch controls the movement of the tube-feet of its 

 radius. In sea-urchins a special set of 

 nerves go to the musculature of the jaws. 



Sense-organs. — The tube-feet are 

 certainly dehcately sensitive to touch in 

 all echinoderms, and in starfishes the ter- 

 minal tentacles seem to be specialized 

 organs of sense. Red " eye-spots " are 

 found at the ends of the arms of star- 

 fishes. These spots are covered with 

 little pits or depressions that are full of 

 pigment and no doubt alDsorb light, but 

 can hardly form a picture on a retina. 

 Over the oral surface of the radii of sea- 

 urchins are scattered modified spines 

 consisting of small spheres resting on a 

 tubercle (Fig. 235). These normally 

 hang straight down ; and if a sea-urchin 

 be turned wrong side up, they press on 

 the skin and perhaps thus make the animal aware of its abnor- 

 mal position. They are thus thought to be organs of " orienta- 

 tion," — informing the animal of its position in the world. 



Fig. 2.35. — One of the 

 little spheres of sea-ur- 

 chins, diat^ramniatieand 

 much magnified : cut 

 lengthwise. (1), the cal- 

 careous mass covered with 

 skin (2), borne on a limy 

 stem (3), which in turn 

 is worked by muscles at 

 (4), over a knob (6) of 

 the limy plates (7) . The 

 muscles are under the 

 control of a nerve-centre 

 at (5). From Lang. 



