162 MANUAL OF ZOOLOGY SECT. 



seen to be continuous with one of a series of little bladder-like 

 bodies, the ampulla (Amp.}, which lie on the other side of the 

 ambulacral ossicles, i.e., in the cavity of the arm. When one 

 of them is squeezed the corresponding 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. 81, rad. amb., 

 Fig. 82, B, Rad. Amb. V., Fig. 84, r\ which appears in 

 the transverse section as a small rounded aperture. From 

 this short side branches (Fig. 84, 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. 81, rad. ne., and 82, Rad. JVv.) of the 

 epidermal nervous system, and is traceable as a narrow thick- 

 ened band running throughout the length of the groove, and 

 terminating in the eye at its extremity, while proximally it 

 becomes continuous with one of the angles of a pentagonal 

 thickening of a similar character, the nerve-pentagon (Fig. 82, 

 Nv.R.), which surrounds the mouth. 



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. 

 This radial blood-vessel, as it is termed, is divided long- 

 itudinally by a vertical septum into two lateral halves 

 (Fig. 8 1, rad.bLv.}. Proximally it communicates with an 



