COELENTERATA AND CTENOPHORA 



standpoint of evolution that a similar body plan occurs as a transitional 

 stage, the so-called gastrula, in the development of most of the higher animals. 

 The widespread occurrence of a hollow, two-layered stage in the ontogeny 

 of metazoans led the great German biologist, Ernst Haeckel (1834-1919), to 

 propose the famous Gastraea Theory. According to Haeckel, both radial and 

 bilateral metazoans pass through a gastrula stage in development because 

 they have descended, by diflferent evolutionary pathways, from a remote 

 common ancestor — a primitive metazoan with the general characteristics of a 

 gastrula, which Haeckel called a gastraea. Unfortunately, it is impossible 

 to prove or disprove phylogenetic theories; the evidence for the existence of an 

 ancestral gastraea is of the same indirect nature as that which supports 

 Haeckel's broader Recapitulation Theory (see p. 225). To many modern 

 biologists the evidence suggests rather that the supposed common ancestor of 

 Radiata and Bilateria was a two-layered form without an internal cavity, 

 somewhat resembling the planula larva of coelenterates and hence referred 

 to as a planuloid or planula-like form (cf. Fig. 7.3, p. 219). Whichever 

 interpretation we follow, the assumption that ectoderm and endoderm are 

 homologous in Radiata and Bilateria implies that these groups originated in 

 a common ancestor. We turn now to a consideration of the simplest of the 

 Bilateria, in the phyla Platyhelminthes and Nemertinea. 



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