PHYLOGENY 



Protozoa must have been independent of that of the rest of the Metazoa. 

 The Porifera are therefore assigned to a separate branch of the subking- 

 dom Metazoa, the branch Parazoa, in contrast to the Eumetazoa. 



The Gastrea Theory. Speculation on the origin of the Metazoa has 

 been dominated by the Gastrea theory of Haeckel, a theory based on the 

 hteral apphcation of the Biogenetic Law, In its original form, Haeckel 

 interpreted the egg as corresponding to an ameboid ancestor, possibly to 

 Amoeba itself. As evidence in favor of this, he pointed to the ameboid 

 eggs of sponges and of some coelenterates. Other types of eggs he as- 

 sumed to be secondary specializations. The egg, of course, undergoes the 

 cleavage divisions which result first in a solid morula and then in a hollow 

 ball of cells, the blastula. The morula was interpreted as corresponding to 

 a simple hypothetical ameboid colony, the Synamoeha, while the blastula 

 was supposed to correspond to a colonial ancestor, the Blastea, more or 

 less comparable to Volvox, but ameboid rather than flagellate. The mod- 

 ern exponents of this theory assume flagellate rather than ameboid an- 

 cestors for the reasons stated above. There being only a single layer of 

 cells in the Blastea, all cell functions were at first shared by all cells, but 

 then a division of labor occurred, with the posterior cells assuming the 

 nutritive functions. These cells then invaginated, with the result that the 

 organism became a two-layered gastrula, having an outer layer of flagel- 

 lated cells ( ectoderm ) and an inner layer of digestive cells ( entoderm ) . 

 Haeckel called this hypothetical organism the Gastrea, and he believed 

 it to be ancestral to all Eumetazoa. Some of the coelenterates he regarded 

 as living gastreads. Next, the Gastrea developed a third cell layer, the 

 mesoderm, between the first two, and he regarded all of the structures of 

 the higher phyla as derived from these three layers. The development 

 of the bottom-feeding habit led to elongation of the body and the forma- 

 tion of primitive worms, similar to the living Turbellaria. From these the 

 higher phyla were developed. 



The Gastrea theory is a beautiful simplification and synthesis of a vast 

 amount of embryological, morphological, and taxonomic data, and it is 

 almost without a serious competitor. As a result, it was, until recently, 

 presented in nearly every elementary textbook of zoology. Unfortunately, 

 however, as Hyman * has pointed out, "it is probably one of those simpli- 

 fications that are too beautiful to be true." As explained in Chapter 3, 

 embryology is not a safe basis for construction of pedigrees, especially if 

 comparison is made between embryos of advanced species and adults of 

 their supposed ancestors, as Haeckel did in this case. At most, embryology 

 should be treated only as one of several corroborative lines of evidence. 

 But there is the further difficulty that even the embryological evidence 

 does not give unequivocal support to the Gastrea theory. For in the coe- 

 lenterates, the group which is closest to the hypothetical Gastrea, gastru- 

 lation ordinarily occurs not by simple invagination of the posterior cells, 

 but by the inwandering of many cells from all parts of the blastula. And 



* By permission from Hyman, L. H., "The Invertebrates," Vol. I, McGraw-Hill Book 

 Co., Inc., 1940. 



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