74 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



FlG. 25. Horizontal Section of Actinia 

 through the stomach, showing septa 

 or mesenteries, and compartments. 



CLASS 3. Actinozoa - 



These marine animals, which by their gay tentacles 



convert the bed of the ocean into a flower garden, or by 



their secretions build up coral 

 islands, have a body like 

 a cylindrical gelatinous bag. 

 One end, the base, is usually 

 attached; the other has the 

 mouth in the center, sur- 

 rounded by numerous hollow 

 tentacles, which are covered 

 with nettling lasso cells. This 

 upper edge is turned in so as 

 to form a sac within a sac, 

 like the neck of a bottle 



turned outside in. The inner sac, which is the digestive 



cavity, does not reach the bottom, but opens into the 



general body cavity 



(Fig. 236). 10 The 



space between these 



two concentric tubes 



is divided by a series 



of vertical partitions 



or mesenteries, some 



of which extend from 



the body wall to the 



digestive sac, but oth- 

 ers fall short of it. 



Instead, therefore, of 



the radiating tubes 



of the Scyphozoan, 



there are radiating 



spaces. No members of this class are microscopic. All 



are long-lived compared with the Hydrozoa, living for 



FIG. 26. Actinia expanded, seen from above, 

 showing mouth. 



