METRIDIUM 61 



5. How many pairs of primary mesenteries, i. e., those at- 

 tached both to the outer body wall and to the actinopharynx, 

 are there? The directive septa are those at the angles of 

 the actinopharynx. The portion of the coelenteric cavity 

 between any two pairs of mesenteries is termed an inter- 

 radial chamber. The space between the two mesenteries of 

 each pair is called an intraradial chamber. 



6. Carefully determine the disposition of the longitudinal 

 retractor muscles on the mesenteries. Do they occupy similar 

 positions on all of the mesenteries? 



7. Examine the upper parts of the mesenteries for open- 

 ings, septal stomata, that put the chambers in communica- 

 tion. 



8. Are the tentacles solid or hollow? 



Make a drawing of a longitudinal section and another of 

 a cross section. Put into these all of the points of the an- 

 atomy you have seen. 



If time and opportunity permit, it is very desirable that 

 this form should be compared with specimens of the order 

 Madreporaria, and later with the Alcyonaria. Such a form 

 as Astrangia may easily be obtained either alive or properly 

 preserved, and will serve to show the relation of the hard 

 parts of the coral to the polyp. You should understand the 

 relation of the septa and the mesenteries, and of the polyps 

 to each other. If specimens are at hand, compare such forms 

 as Orbicella, Favia, and Meandrina, or any forms that show 

 gradations from separate calices to fused groups, and under- 

 stand the positions of mouths, the arrangement of the coelen- 

 teric chambers, and the way in which the colony has come 

 to its present form. You should also examine large branch- 

 ing colonies and determine why branches are formed and 

 how they arise. 



Examine the structure of an alcyonarian colony and see 

 how the polyps are placed. The structure of the expanded 

 polyps is nicely shown by Renilla. The spicules of such 

 forms as Gorgonia may be obtained by boiling a portion of 



