412 COMPARATIVE ANATOMY CHAP. 



(a) That the body epithelium stretching across the ambulacral fur- 

 row is thickened into the longitudinal ridge which stands up 

 in the base of the furrow, and here contains : 



(/;) The radial nerve strand (lying in the epithelium itself) (4). 



We find further : (c) below the latter, to the right and left, the 

 trunks of the deeper oral nervous system (1). 



(d) The radial pseudohafimal canal (5), which is divided into two 

 lateral portions by a vertical septum. 



FIG. 355. Transverse Section through the arm of an Ophiuroid. diagrammatic. 1, Ambu- 

 lacral tentacle; 2, its water vascular canal ; 3, epineural circular canal at the base of the tentacle ; 

 4, circular ganglion at the base of the tentacle ; 5, ventral shield ; 6. radial epineural canal ; 7, radial 

 nerve trunk of the superficial oral nervous system ; 8, continuation of the axial organ in the arm (?) ; 

 0, radial trunk of the deeper oral nervous system; 10, radial pseudoluemal canal; 11, peripheral 

 branch of the radial nerve trunk; 12, spine; 13, lower (oral) iiitervertebral muscle cut across ; 14, 

 lateral shield; 15, vertebral ossicle; 16, upper (apical) intervertebral muscle; 17, dorsal canal of 

 the brachial cavity (crelom) ; 18, ciliated strip ofendothelium ; 10, dorsal shield ; 20, radial canal of 

 the water vascular system; 21, lateral portions of the brachial cavity, which are segmentally re- 

 peated ; 22, branch of the water vascular system running to the tentacle ; 23, ganglion at the base 

 of the spine ; 24, motor branch of the nerve (of the deeper oral system). 



(e) The radial canal of the water vascular system (2), with the 

 canals of the tube-feet branching from it. (All these are 

 separated from one another by thin layers of connective 

 tissue.) 



(/) The ambulacral plates (30), with the transverse muscles which 

 connect them (28). 



(g) Still further in, and projecting into the body cavity, are the 

 ampullae (26) of the tube-feet. 



