372 ZOOLOGY SECT. 



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 ambulacra! 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 backwards through the canal being prevented by the 

 closing of the valve. 



Vascular and Nervous Systems. Running along the 

 ambulacral groove, immediately below where the ambulacral 

 ossicles of opposite sides articulate, is a fine tube, the radial ambu- 

 lacral vessel (Fig. 308, rad. amb), which appears in the transverse 

 section as a small rounded aperture. From this short side-branches 

 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 (Fig. 308, rad. ne) of the epidermal 

 nervous system, and is traceable as a narrow thickened 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. 309) 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 nerve-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 system : like the epidermal, the deep nervous system has a 

 central part in the form of a pentagon, which in this case is double, 

 surrounding the mouth. A third set of nerve elements (the aboral 

 or coelomic nervous system) extends along the roof of the arm super- 

 ficial to the muscles. 



The two radial nerve-bands of the deep nervous system are 

 thickenings of the lining membrane of a space overlying the 

 radial nerve and underlying the radial ambulacral system. This 

 space (rad. bl. v.), extending, like the other parts that have been 

 mentioned, throughout the length of the arm, forms part of a 

 system of channels, the perihcemal system, which have been regarded 

 as constituting a blood-vascular system. This radial perih&mal 

 vessel or sinus, as it is termed, is divided longitudinally by a vertical 

 septum (sept.) into two lateral halves. Internally it com- 

 municates with an oral ring-vessel surrounding the mouth 

 and likewise divided into two by a septum. The inner division 



