350 



ZOOLOGY 



SECT, 



(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 

 radial ambulacral vessel (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 rad'u'l nerve i 

 (Fig. 277, rad: ne.) 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 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 (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 nerv( 

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

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

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

 radial parts of the deep nervous system : like the epidermal th( 



Fi<:. 278. Ambulacral system of a Starfish. 

 -(. ampullae ; rtp.Polian vesicles; c. circular 

 canal ; m. madreporite ; 'm', madreporic 

 canal ; t. tube-feet ; p, radial vessels ; /, 

 branches to ampullae. (After Gegenbaur.) 



