336 



ECHINODERMATA 



[CH. 



the radial perihaemal canals and are outgrowths fiom the 

 coelom. From their upper walls are derived the muscles which 

 move the ambulacral ossicles on one another: from their lower walls 

 a layer of ganglion cells and nerve-fibres is developed, which may 

 be termed the coelomic nervous system in order to distinguish 

 it from the main mass of ganglion cells and fibres which are derived 

 from the ectoderm. This coelomic nervous system, which is very 

 thin in the star-fish, seems to serve as the channel by which 

 impulses from the radial nerve-cord reach the ambulacral muscles. 

 The five pairs of radial perihaemal canals are connected with one 



B. 



FIG. 154. Pedicellariae from Asterias glacialis. From Cue"uofc. 



Crossed form x 100. 1. Ectoderm. 2. Base of left "jaw." 3. Muscle 

 closing the "jaws." 4. Basal ossicle. 5. Muscle opening the 



"jaws." 6. Fihrous hand connecting the basal ossicle with one of the 

 rods of the skeleton. 7. Fibres of peduncle. 



Straight form. 1. Basal ossicle. 2. "Jaws." 3. Adductor 



muscle. 4. Muscle closing the "jaws." 5. Muscle opening the 



"jaws." 



another by a circular canal lying above the nerve-ring called the 

 outer perihaemal ring. Inside this is another circular canal 

 called the inner perihaemal ring, which is an expansion of the 

 foot of the axial sinus (see p. 338). 



The upper or aboral surface of the star-fish is provided with two 



. most interesting groups of organs, pedi cellar iae and 



dermal branchiae. The former are minute pincers, 



composed of two or rarely three blades moving on a basal piece. 



These close when the skin of the back is irritated; their main 



purpose appears to be to keep the surface of the animal clear from 



