30 



the whole inimber of arms, as is shown by the partial or 

 complete failure of co-ordination when the commissure 

 is injured or wholly destroyed; while the commissure and 

 chiasma which connect the bifurcating nerve cords in the 

 second primibrachial in like manner co-ordinate the 

 movements of each pair. The brachial cords are the 

 paths along w^hich afferent and efferent impulses travel 

 to and from the central capsule. If the cord only be 

 severed at any one point of an arm, leaving the 

 ambulacral nerve band intact, even the severest irritation 

 applied to the distal portion of the arm beyond the injury 

 fails to excite any response in the disc and the remaining 

 uninjured arms. Irritation applied to the cut end of the 

 cord in the proximal part of the arm causes immediate 

 and strong flexion of that and all the uninjured arms, 

 while irritation applied to the cord in the distal part of 

 the arm causes similar movements in that part of the arm 

 alone. 



The Genital Organs. 



The genital organs, testes and ovaries, are specialised 

 portions of a system of sterile cords which radiate from 

 the disc and traverse the arms and pinnules. In a 

 transverse section of an arm (PI. IV., fig. 41) there may 

 be seen in the horizontal partition which divides the 

 coeliac from the subtentacular canals another rounded 

 coelomic space, lined like its neighbours with endothelium 

 {gen. Ic). This is the genital lacuna or sinus. It encloses 

 another tube, the genital strand or rachis {gen. ra.), which 

 is suspended to its walls by slender filaments of connective 

 tissue. At first a solid cord, the rachis eventually 

 becomes a hollow tube lined with germinal epithelium. 

 In its course along the arm it supplies a branch to each 

 ^jinnule with the exception of the first. In the pinnules 



