190 THE ORGANIZATION 



respiratory branches. This office is performed by the introduc- 

 tion of water through the posterior opening of the intestine into 

 the tapering cloaca (g- 3 ) and thence into the branches, where, by 

 the enormous expansion of their branchlets, an extent of res- 

 piratory surface is bathed that is scarcely equalled in any other 

 known group of animals. 



The nervous system consists of a simple flat collar (fig. 114, c) 

 which embraces the throat almost close up to the edge of the 

 mouth, and five slender threads which extend along the sides of 

 the body just beneath the thick skin (fig. 116, s). The posi- 

 tion is indicated by these longitudinal rows of dashes, (fig. 114,) 



, of which one trends along the lower mid-line (aq*) 



of the body, and two along each side, right and left (aq 2 , aq 6 ). 

 In the foreshortened view these five nearly equidistant points 

 (fig. 115, aq*, aq s , aq g ) indicate their position. 



The aquiferous circulatory system is much more complicated 

 than in the starfish. But in order that you may the better 

 understand its distribution I must first state that the throat is 

 encased in a sort of sheath, which consists of fifteen calcareous, 

 stave-like bodies (fig. 114, f,f 1 } joined side by side like the staves 

 of a pail. They are usually designated as the buccal plates. 

 Of these, ten, in pairs, at alternate intervals with the remain- 

 ing five, extend backward beyond the latter, in the form of 

 five broad forks (/). Their several positions right and left corre- 

 spond with the five longitudinal nervous threads, and they serve 

 as solid bases of attachment for five pairs of ribbon - shaped 

 muscles, (fig. 115, ms, ms l , ms 2 , fig. 116, w, wz 1 ,) which extend 

 from just behind the base of the tentacles, along the sides of 

 the body ; one of each pair running on each side of the line along 

 which the five nervous threads trend. The general lining of the 

 body is a muscular layer (fig. 116, f,f 1 ,) which lies immediately 

 within the outer skin (s), and consists of fibres which bind the 

 body as with innumerable, circular, contractile hoops. Along the 

 inner face of this layer the five pairs of longitudinal muscles 

 (m, m l ) are attached, and along its outer face, next the skin, the 

 five nervous threads (n) extend in so many lines, exactly over the 



