174 AN INTRODUCTION TO ZOOLOGY. 



Examine the nerves of the common Starfish, which 

 may be seen by pushing aside the tube-feet near the 

 middle line of the rays. They form the central line 

 seen on the under surface of each ray. They are not 

 cords of tissue like the internal nerves of other animals, 

 found on dissection ; but they are outside and flat. 

 They are covered only with a very thin layer of epi- 

 thelium, and maybe regarded as mere thickenings of 

 the skin. In other words, ive have here the primitive 



c^Vi- ^^^ 



Pig. 41.— Position or nerve in arm of starfish, as seen in transverse section : 

 c, cuticle ; sic, skin ; n, nerve lying next the skin : v, section of vessel ; u, section 

 of vessel belonging to the water- vascular system. 



form of nervous tissue derived from the outside body- 

 layer {ectoderm = epiblast), and not yet tucked in and 

 separated from its parent tissue, as in other forms. 

 The same is the case with those of the Jelly-fish ; but 

 those of the Starfish are here especially mentioned 

 because the animal is so common as to be familiar 

 to every one. 



The BcHiNOiDEA, or Sea-urchins, receive their name 

 from the fact that they are covered with spines. 

 These spines are movable, and in the common Sea- 

 urchin, which has rather large ones, they may easily 



