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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



invagination. A common substitute is the multiplication of cells 

 at some point on the inner surface of the blastoderm, followed by 

 an inward migration of these cells to form a stomach that never 

 was a part of the exterior animal. This plan of stomach building 

 is distinguished as gastrulation by proliferation. But in general 

 invagination and proliferation are interchangeable processes in 

 embryonic development, so it is nothing to worry about if an animal 

 in its growth adopts either one in place of the other. Where gastru- 

 lation takes place by proliferation there is, of course, no blastopore, 

 though there is sometimes an external depression opposite the area 

 of internal proliferation, suggesting that this is the site of a formerly 

 open ancestral blastopore, which has simply been closed to accom- 



PiGDRB 5. — The fundamental structure of coelenterates. 



A, the polyp type. B, the jellyfish, or medusa, type. 



Ecd, ectoderm ; End, eudoderm ; Qc, gastrocoele ; Mth, mouth ; Tl, tentacle. 



modate the conditions of development. An animal with a closed 

 blastopore eventually acquires one or two secondary openings into 

 the stomach. 



At this point it is of interest to note that the Coelenterata have 

 never evolved beyond the 2-layer stage of structure. They consist 

 essentially of an outer ectodermal body wall, and an inner endoder- 

 mal stomach wall (fig. 5 A), though there may be a few accessories 

 such as nerves, motor tissue, and stinging organs, all developed 

 from the ectoderm. The coelenterates are either cylindrical animals 

 attached by one end to a support (A), or they are disk-shaped and 

 float free in the water (B). Those of the first kind are known as 

 polyps, the second are in general termed jellyfish, or medusae. In 

 each form there is a mouth located axially; in the polyps it is at 

 the free end of the body (A, Mth), in the jellyfish it is at the end 



