80 THE MOST PRIMITIVE METAZOA 



It is interesting to compare the above statement with one taken 

 from Hyman (1940). " The contrary theory, that the ancestral 

 coelenterate was a primitive medusa, therefore seems more 

 acceptable. This could readily have developed from the meta- 

 gastraea by putting forth tentacles and w T hen armed for food 

 capture would not have been limited to a bottom habitat." R. C. 

 Moore (1956) is of a similar opinion to Hyman. " Next the 

 conclusion that the polypoid and medusoid types of organisation, 

 instead of representing a more or less unexplained ' alternation of 

 generations ' constitute the products of evolutionary differentia- 

 tion in which the polypoid form is a persistent early growth, and 

 the medusoid is the normal adult type of coelenterate, leads to the 

 interpretation of medusoids as the initial type of coelenterate. This 

 is consistent with the paleontological record, which includes 

 numerous Lower Cambrian and even Precambrian medusoid 

 fossils. Consequently the simplicity of the hydroid forms is not 

 accepted as a basis for placing them in first position among 

 various types of coelenterates. Precedence is assigned to early 

 medusoids." 



Though the medusa has been suggested as the basic form in the 

 Coelenterata this has not been followed up by claiming that the 

 Scyphozoa are the most primitive class of the Coelenterata. The 

 life cycle of the Scyphozoa with their dominant medusa and their 

 temporary polyp (hydratuba) might fit in with the primitive system. 

 The Stauromedusae such as Haliclystus and Lucernaria indicate 

 the way in which an adult polyp, even a highly specialised polyp, 

 could have arisen. The nematocysts in the Scyphozoa are more 

 limited in range of form than those in the Hydrozoa. It might 

 be objected that the medusae of the Scyphozoa are very much 

 more complex than those of the Hydrozoans, but the complexity 

 of the present-day forms does not mean that the original forms were 

 of the same complexity. The present-day forms and even the 

 Cambrian fossils have a tremendous history of development 

 behind them. The choice of a primitive class in the coelenterates 

 will clearly depend upon the light that such a choice throws on our 

 understanding of coelenterate morphology. 



The most popular choice of primitive class in the coelenterates 

 seems to lie between the Hydrozoa and the Anthozoa. Opinion is 

 divided as to which of the Hydrozoa are the most primitive. Thus 



