III. 



Special description of the organs. 



(Organology). 



I. The Skeleton System. 



(Tab. IV. tig. 1—22 and Tab. V). 



The solid support for the disc, as well as for the arms, is composed of numerous 

 calcareous pieces more or less firmly connected with each other, and forming together the 

 so-called ambulacral skeleton. On the disc this skeleton forms a narrow exterior frame or 

 ring (fig. 1, 2 & 3) consisting of calcareous pieces immovably connected; and to the outer 

 side or periphery of this ring the ambulacral skeleton of the arms is attached. The latter 

 skeleton (see fig. 15 — 22), which occupies the lower side of the arms from the base to the 

 extreme point, consists of numerous sections or joints connected, to a certain extent mov- 

 ably, with each other by elastic bands, and forming the so-called ambulacral vertebrae, of 

 which each is again composed of a certain number of symmetrically arranged calcareous 

 pieces. Both the ambulacral skeleton of the arms and that of the disc are externally 

 covered with a smooth tendinous skin, like a thin envelope, firmly attached by growth, or 

 as it were a kind of periosteum, which can only be entirely removed by long maceration or 

 by employing chemical reactives. We shall now regard the ambulacral skeleton of the arms, 

 and that of the disc each for itself. 



a. The ambulacral Skeleton of the arms. 



(Tab. IV, fig. 15-22). 



As already noticed, the ambulacral skeleton of the arms consists, like that of the 

 other star-fishes, of a series of similar successive sections or joints which have been called 

 vertebrae by reason of a certain general resemblance which the skeleton of the arm exhibits 



