17 



formed by both cell layers, became the more immediate pro- 

 genitors of the Coelenterata. The organism* which occupied 

 the point where the Coelenterate branch first divided was 

 probably a short, wide, sac-like form, fixed by its aboral end, 

 and having large tentacles in multiples of four placed equa- 

 torially, and possibly also smaller tentacles around the mouth 

 opening (fig. 12, A). This form, probably, on becoming 



Fig. 12. Primitive Hydrozoa. Three diagrammatic vertical sections (after 

 Lankester), A, Ancestral Hydrozoon shewing a condition intermediate between 

 the Hydra-form and the Medusa-form. B, Simple Hydra-like ancestor produced by 

 modification of A. C, Simple Medusa-like ancestor produced by modification 

 of A. m, mouth; en, enteron; t, tentacle; g, reproductive organs. A, Indicates 

 the same point in each form, viz., the base of the tentacle. In A it is at the 

 equator of the globose body, in B at the top of the cylindrical body, and in C at 

 the margin of the beU-shaped body. The dark inner wall of the enteron is the 

 endoderm, the lighter outer layer of the body is the ectoderm. 



sexually mature detached its base, and, by enlarging its 

 equatorial region into a disc, and finally into a bell concave 

 on the oral face and provided with circularly-placed bands 

 of muscle, acquired a swimming organ by the contractions 

 of which it could be propelled through the water (see 

 fig. 12, C). The ova and spermatozoa were then developed 

 upon the inner walls of the enteric cavity of this Medusa- 

 like form. 



This common ancestor of all the Coelenterata (which is 

 most nearly represented at the present day by the Hydra- 



* See Ray Lankester's article, "Hydrozoa," in Ency. Brit., 9th edition, 

 p. 552. 







