162 MANUAL OF ZOOLOGY sect. 



a series of little bladder-like bodies, the ampulla, which lie 

 on the other side of the ambulacral ossicles, i.e., in the 

 cavity of the arms. When one of them is squeezed the cor- 

 responding tube-foot is distended and protruded, the cavities 

 of the tube-foot and the ampulla being in communication 

 by means of a narrow canal running through the ambulacral 

 pore ; and it is in this way that the foot is protruded in the 

 living animal. The corresponding ampulla being contracted 

 by the contraction of the muscular fibres in its walls, the 

 contained fluid is injected into the tube-foot and causes its 

 protrusion. 



Running along the ambulacral groove, immediately below 

 where the ambulacral ossicles of opposite sides articulate, is 

 a fine tube, the radial ambulacral vessel (Fig. 87, rad. amb ; 

 Fig. 88, B, Rad. Amb. V; Fig. 90, ;-), which appears in 

 the transverse section as a small rounded aperture. From 

 this short side branches (Fig. 90 r) pass out on either side 

 to open into the bases of the tube-feet. Below the radial 

 ambulacral vessel is a median thickening of the integument 

 covering the ambulacral groove ; this marks the position of 

 the radial nerve (Figs. 87 and 88, Rad. ne) of the epidermal 

 nervous system, and is traceable as a narrow thickened band 

 running throughout the length of the groove, and terminat- 

 ing in the eye at its extremity, while internally it becomes 

 continuous with one of the angles of a pentagonal thickening 

 of a similar character, the nerve-pentagon, which surrounds 

 the mouth. 



The two radial nerve-bands of the deep nervous systems 

 are thickenings of the lining membrane of a space overlying 

 the radial nerve and underlying the radial ambulacral system. 

 A channel throughout the length of the arm above the 

 radial nerve forms part of a system of channels which are 

 usually regarded as constituting a blood-vascular system. 



