488 ECHINODERMATA — OPHIUROIDEA chap. 



takenly on another cavity, which will be mentioned in connexion 

 with the genital organs. 



The radial perihaemal spaces of the arms open into a " peri- 

 haemal ring " representing the outer perihaemal ring of Asteroids ; 

 but the axial sinus does not have any such extension as consti- 

 tutes the inner perihaemal ring in Starfish. So-called oral circular 

 and radial blood strands are to be found in similar positions to 

 the corresponding structures in Asteroidca. 



The nervous system might have been expected to have 

 become very much modified, since the activities of the Brittle Stars 

 are so different from those of the Starfish. It is indeed a 

 universal rule in the Animal Kingdom that, concomitantly with 

 the increase in size and activity of a muscle, there is a corre- 

 sponding increase in the number of ganglion-cells which control it. 

 An accurate radial section of an arm shows that there is, corre- 

 sponding to the interspaces between the two vertebrae, a ganglionic 

 swelling of the nerve-cord. As in Asteroids, there are not only 

 ectodermic ganglion -cells on the under surface of the cord 

 abutting on the epineural canal, but also coelomic ganglion-cells 

 derived from the floor of the radial perihaemal canal. Both these 

 categories of cells are largely increased in number in the ganglion. 

 From the dorsal -cells arise a pair of large nerves which pass 

 directly up and supply the great intervertebral muscles. From 

 the interspace between the ganglia a direct prolongation of the 

 ventral part of the nerve-cord, the so-called pedal nerve, extends 

 out along the side of the tentacle, as in Asteroids. In Ophiuroids 

 it swells out into a ganglion, completely surrounding the tentacle 

 and giving off nerves to the surfaces of the arm which terminate 

 in the cuticle. 



There is a large ganglion where the radial cord joins the 

 nerve-ring, and, owing to the more specialised condition of the 

 nervous system, a severed arm in an Ophiuroid is much more 

 helpless than an arm of an Asteroid. It will not carry out 

 " escape movements," and is for a long time rigid under the 

 shock of section ; at last it simply gives reflex movements on 

 stimulation. 



Preyer ^ endeavoured to test the " intelligence " of Ophiuroids 

 by observing how they would adapt themselves to circumstances 

 which it might be fairly assumed they had never encountered 

 ^ "Bewegungen d. Seesternen," Mitth. Zool. Stat. Ncapcl, vii. 1886-87, ji. 123. 



