32 CCELENTERATA. 



and the mesenteries, and of the polyps to each other. If speci- 

 mens are at hand, compare such forms as Orbicella, Favia, and 

 Meandrina, or any forms that show gradations from separate 

 cahces to fused groups, and understand the positions of mouths, 

 the arrangement of the coelenteric chambers, and the way in 

 which the colony has come to its present form. You should 

 also examine large branching 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 nicel}^ shown by Renilla. The spicules of such forms 

 as Gorgonia may be obtained by boiling a portion of a colony 

 in caustic potash. What purpose do such spicules serve? 



CTENOPHORA, 



MNEMIOPSIS. 



This form belongs to the group of animals popularly called 

 '^ comb-jellies," and occurs along the coast in irregular abun- 

 dance during the summer months. Specimens are very phos- 

 phorescent when disturbed, so, when they are abundant, the 

 display caused by them while rowing at night is sometimes bril- 

 liant. They may frequently be seen during the daytime and can 

 often be satisfactorily observed in the shade of a wharf when the 

 water is calm. 



Unmutilated, living material can be studied to best advan- 

 tage, but preserved material may be had that is quite satisfac- 

 tory for anatomic study. 



1. In general appearance a specimen resembles a hydrozoan 

 medusa, with its ahoral surface elongated until, as a whole, it 

 approaches the shape of a fowl's egg. 



2. The broader or oral end bears two heavy terminal lobes, 

 between the bases of which is the slit-like 7nouth. We may con- 

 sider the elongation of the mouth to be in the antero-posterior 

 line. A bilateral symmetry is thus evident. 



3. On each lateral surface of the animal, midway between 



