SURVEY OF INVERTEBRATES 



75 



wintering in this form, however, the fixed individual proceeds, as 

 it were, to slice itself up in orderly fashion into a series of saucer- 

 like embryo medusae which, one by one, separate, swim away, 

 and eventually attain the adult, sexual Aurelia form. (Fig. 40.) 



EndodernreS 



Stomach 

 Sense 

 tentacle 



Endoderm ■ 



Fig. 40. — Aurelia, life-history. A, B, C, longitudinal sections through 

 gastrula stages; D, scyphistoma; E, F, strobila; G, H, ephyra, I, vertical 

 section through adult. A, B, and C are more highly magnified than the other 

 figures. (From Hegner, after Kerr.) 



Anthozoa. So far our survey of the Coelenterata has shown that 

 all are polyp-like animals, and the final class to be considered, the 

 Anthozoa, is no exception, comprising, as it does, the Sea Anemones 

 and the Corals. 



Metridium, the well known Sea-anemone of the North Atlantic 

 coast, is common in tide pools or attached to piers of wharves; 

 its slight powers of creeping being seldom exerted. It is apparently 

 a ruggedly built polyp, nearly cylindrical in form, with the upper 

 end disclosing the mouth in the midst of a crown of numerous 

 tentacles. (Fig. 41.) 



The mouth opens into a large gullet which leads down into 

 the enteron. The latter is quite complex because its walls give 

 rise to several series of vertical, radially arranged partitions, or 

 mesenteries, thereby increasing the functional digestive and 

 respiratory surface. Within the enteric cavity are peculiar, long, 



