350 



ZOOLOGY 



SECT. 



1 



(amp. Figs. 277 and 280 : ap, Fig. 278) 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 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 their walls, the con- 

 tained fluid is injected into 

 the tube-foot and causes its 

 protrusion. 



Vascular and Nervous 

 System. Running along the 

 ambulacral groove, immedi- 

 ately below where the ambu- 

 lacral ossicles of opposite sides 

 articulate, is a fine tube, the 

 radlt/l ainlitdacriil vexxcl (Fig. 

 277, rad. amb, Fig. 278, r), 

 which appears in the trans- 

 verse section as a small rounded 

 aperture. From this short side- 

 branches (r' } Fig. 278) 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 ratlial nerve 

 (Fig. 277, rad. ne.) of the 

 epidermal nervous si/stc'm, 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 internally it becomes 

 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. 279) the ventral median thickening, or 

 radial nerve (rail, 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 nervr- 

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

 constituting the radial nerve, is a band of cells (d. nerv.) also of a 

 nervous character. These more deeply placed nerve-bands are the 

 radial parts of the deep nervous si/xtc'/n : like the epidermal the 



(; . -J7S. Ambulacral system of a Starfish. 

 it. :nn]iull;e ; nf>. I'olian vesicles; c. circular 

 canal ; a,, ruadreporite ; m', madreporic 

 canal ; t. tube-feet; p, radial vessels; /', 

 branches to ampulla;. (After Gegenbaur.) 



