280 ELEMENTS OF COMPARATIVE ZOOLOGY. 



cult to demonstrate this in the preserved specimen. Notice 

 the openings into the branchiae. 



Eemove the hepatic caeca from one arm and find the 

 lobular reproductive organs near the base of the ray. Where 

 does this duct connect with the external wall ? Would you 

 consider this point (at which the duct opens to the exte- 

 rior) as radial or interradial ? 



Below (that is, oral to) the pylorus is the cardiac portion 

 of the stomach, produced into gastric pouches in each of 

 the rays. Trace from these pouches the thin retractor 

 muscles into the ray to their attachment to its floor. 



Make a sketch of your dissection, showing in the centre 

 the stomach, in one arm the hepatic caeca, in a second the 

 reproductive organs, a third with cardiac retractors and am- 

 pullae, a fourth with the dorsal surface, and leave the other 

 arm for structures, to be added later. 



Carefully cut away stomach a little inside the mouth, 

 and then trace the stone-canal (a hard S-shaped tube) 

 downward from the madreporite to the region around the 

 mouth. Examine this circumoral region from the aboral 

 side, and find the ten Polian vesicles (much like the ampul- 

 lae) and, inside of these, the small sacculated racemose vesi- 

 cles. How many are there of these ? What do you find 

 in the place of the one needed to make symmetry ? Beside 

 the stone-canal is a thin-walled sac, the so-called heart. 

 Sketch the organs in this paragraph, and keep the drawing 

 for further additions. 



Remove the ampullae, membranes, etc., from the floor of 

 one of the rays and see the ambulacral plates which meet 

 in the median line. Notice the openings in this ambulac- 

 ral area by means of which the ampullae connect with the 

 ambulacra. Are these ambulacral pores in or between the 

 plates ? How many rows of them do you find in an arm ? 



