Chap, xii.] NERVOUS SYSTEM OF CRINOIDS. 



409 



a.' 



the nerve in the arm of a starfish, and it has similar 

 relatio'ns to a nerve ring which runs round the mouth. 

 If we now look at the opposite side of the section, we 

 find another and larger 

 cord which gives oft 

 branches to the muscles 

 of the arms (a a) ; this 

 cord, if it be followed 

 up, will be found to 

 end in an organ, the 

 so - called " chambered 

 organ," which lies in 

 the centred orsal piece 

 (see page 292) of the 

 Crinoi('. Now, if the 

 visceral mass, part of 

 which is the circumoral 

 nerve ring, be alone 

 removed, the arms will 

 continue to move as 

 regularly as they did 

 before, and the Crinoid 

 will still be able to 

 swim about in the 

 water. If, on the other 

 hand, the five - cham- 

 bered organ be stimu- 

 lated, then, as Dr. 

 Carpenter has shown, 

 there is a sudden and 

 simultaneous flexion of 

 all the arms. The ex- 

 istence of these two 



apparently independent nerve systems in a Crinoid 

 is a difficulty which the morphologist has not yet 

 been able to solve, but the anatomical and physio- 

 logical evidence in favour of the nervous nature 



CL 



Fig. 177. Cross Section of a Pin- 

 ule of the Arctic Feather- 

 star (Antedon (schncJiti) ; x 75. 



a, Axial cord ; a', its branches ; ag, ambu- 

 lacral groove; b, radial blood- 

 vessel ; gv, genital vessel ; ov, ovary ; 

 n, radial nerve ; pj, pinnule joint ; 

 w, water-vessel ; T, tentacle. (From 

 Carpenter, altered from Ludwig.) 



