PHYLUM ECHINODERMATA 



379 



(Figs. 302 and 305, mnp. ; Fig. 303, «^)— are arranged like the tube- 

 feet, in a double row on each side, a higher row and a lower, 

 there being one opposite each ambulacral pore. When one of 

 them is squeezed, the corresponding tube-foot is distended and 

 protruded, the cavities of the tube-foot and ampulla being in 

 communication by means of a narrow canal running through the 

 ambulacral pore and provided with a valve. 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, 

 the return of the water back- 

 wards through the canal being 

 prevented by the closing of 

 the valve. 



Vascular and Nervous 

 System. — Kunning along the 

 ambulacral groove, immedi- 

 ately below where the ambu- 

 lacral ossicles of opposite sides 

 articulate, is a fine tube, the 

 radial amhidacral vessel (Fig. 

 302, rad. amb, Fig. 303, r), 

 which appears in the trans- 

 verse section as a small rounded 

 aperture. From this short side- 

 branches (Fig. 303, 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 am- 

 bulacral groove: this marks 

 the position of the radial nerve 

 (Fig. 302, rad. ne) of the 



epidermal nervous system, and is traceable as a narrow thickened 

 band running throughout the length of the groove, and ter- 

 minating in the eye at its extremity, while internally ; it be- 

 comes continuous with one of the angles of a pentagonal 

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

 surrounds the mouth. In thin sections (Fig. 304) the ventral 

 median thickening, ov radial nerve {rad. nerv.), as well as the nerve- 

 pentagon, are seen to be thickenings of the epidermis, consisting 

 of numerous vertically-placed, fibre-like cells, with their nuclei at 

 their outer (lower) ends, intermixed with longitudinal nerve-fibres 

 and with nerve-cells. Above this, on each side of the epidermal 

 nerve-thickening constituting the radial nerve, is a band of cells 



Pig. 



—Ambulacral system of a Starfish. 

 a. ampullte ; ap. Polian vesicles ; c. circular 

 canal ; m. madreporite ; m', madreporic 

 canal ; t. tube-feet ; p. radial vessels ; r'. 

 branches to ampuUse. (After Gegenbaur.) 



