ASTERIAS VULGARIS. 43 



Exercise 5. Copy the outline of the starfish in Exercise i and fill in the 

 details of structure indicated above, where necessary showing some 

 in one arm, some in another. Omit the madreporite. 



Technical Note. In a specimen that has had the water vascular system 

 injected remove all the organs already figured. Study under water, 

 as before. 



AMBULACRAL SYSTEM: 



a) Along each side of the ambulacral ridge will be seen a double row 



of the bulb-like ampullae already mentioned. These communicate 

 with the radial water tubes to which reference has also been made. 



b) In the bony frame-work about the mouth runs a circular water 



ring from which the radial water tubes branch off. This ring 

 also sends up, at points between the origins of the water tubes, 

 two sets of ampullae-like bodies the Polian vesicles, of which 

 there are ten, and, just below them, the Tiedemann bodies, 

 nine in number. 



c) The circular water ring is connected with the madreporite by an 



S-shaped tube, the stone canal. Thus the ambulacral system 

 madreporite, stone canal, circular water ring, radial water tubes, 

 ampullae, and tube feet is supplied with water for the hydraulic 

 operations incident to locomotion. 



Exercise 6. Using the outline in Exercise i, as before, make a semi- 

 diagrammatic drawing of the ambulacral or water vascular system. 



Exercise 7. Make a study of the physiological processes correlated 

 with the various sets of organs you have just represented by drawings, 

 and enter results in your notes. 



HINTS ON COLLECTING. It is a simple matter to prepare the dry 

 specimens wanted for a study of the external anatomy. The starfish 

 are collected in shallow water along the shore and placed in a pail of 

 fresh water for about a half hour. This will kill them, without allowing 

 the body to collapse. After this treatment they may be placed hi 

 alcohol for about the same length of time and then spread in the hot 



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