2. The circular canal, which is joined by the stone canal at the 

 outer margin of the peristomal membrane, follows the margin of 

 the membrane and thus encircles the mouth. Originating from 

 it at points very near the ampullae of the first tube feet are nine 

 small vesicles, Tiedemann bodies. They are smaller than the am- 

 pullae and project toward the mouth. The position where the tenth 

 Tiedemann body might be expected is taken by the stone canal. 



3. Leaving the circular canal are five radial water tubes, one 

 for each arm. These tubes lie along the oral surfaces of the 

 ambulacral plates, and are accordingly not visible on the inside of 

 the animal. The position of the tube can be best understood by 

 making a transverse section of the arm. It will then be seen either 

 in injected or uninjected specimens, lying immediately below the 

 ambulacral plates. In injected specimens it may be followed by 

 dissecting on the oral side, from the circular canal to the ex- 

 tremity of the arm where it ends in a small tentacle. 



4. Along the sides of the ambulacral ridges, within the body 

 cavity, are rows of little bag-like ampullae. Determine their relation 

 to the ambulacral pores. In a dissection it is hard to find the 

 connecting tubes which join the radial tubes to the tube feet, but 

 they can sometimes be seen in sections of the arms of injected 

 specimens. They can readily be seen in microscopical preparations. 



The wa'er vascular system is very distinctive for the echinoderms, 

 and you should understand perfectly: (i) How the tube feet are 

 extended. (2) AYhat causes them to adhere. (3) The connection 

 between tube feet, ampullae, connecting canals, radial water tubes, 

 circular canal, stone canal and madreporite. (4) How it is possible 

 to extend one foot without extending others. 



Make a drawing showing the arrangement of the water vascular 

 system. 



IV. NERVOUS SYSTEM. 



This is not easily studied by dissection. It consists of a nerve 

 ring which encircles the mouth, and lies just ventral to the circular 

 water canal, and five radial nerves that extend down the arms just 

 beneath the radial water tubes, to end at the tips of the arms in 

 pigment spots. The whole central nervous system is superficial 

 and forms a portion of the outer covering of the body. The radial 

 nerves can be seen by separating the rows of ambulacral feet, but 

 it is much more satisfactory to study them in prepared sections. 



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