412 



GOMPARA TIVE ANA TOMY 



CHAP. 



(a) That the body e})ithelium stretching across the ambulacral fur- 

 row is thickened into the longitudinal ridge which stands up 

 in the base of the furrow, and here contains : 



{h) 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 pseudoha;mal canal (o), which is divided into two 

 lateral portions by a vertical septum. 



Ki(i. 355.— Transverse Section through the arm of an Ophiuroid, diagrainmatic. 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 tlie axial organ in the arm (?) ; 

 9, radial trunk of the deeper oral nervous system; 10, radial pseudoha^mal canal; 11, peripheral 

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

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

 the brachial cavity (cojloni) ; 18, ciliated strip of endothelium ; 19, dorsal shield ; 20, radial canal of 

 the water vascular system ; 21, lateral portions of the brachial cavity, which are segnientally i-e- 

 peated ; 22, branch of the water vascular system running to the tentacle ; 28, ganglion at the base 

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



(c) The radial canal of tlie 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). 



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

 ampullar (26) of the tube-feet. 



