GENERAL ZOOLOGY 



some degree the so-called gastrula stage in their life cycles may be taken to 

 indicate that a two-layered condition, which now persists in the adult stage 

 in the simplest coelenterates and as a developmental stage in higher animals, 

 was characteristic of the common ancestors of Radiata and Bilateria. What- 

 ever its evolutionary significance, the gastrula is morphologically a two- 

 layered sac, generally comparable with the basic type of structure in simple 

 coelenterates. Similar interpretations may be based on the appearance of 

 fish-like stages in the development of amphibians and higher vertebrates, and 

 on the existence of various stages of development within other phyla which 

 are suggestive of ancestry. It should be emphasized, however, that no existing 

 species of animal is regarded as the specific ancestor of any higher group. 

 The grand course of evolution can be pictured if we speculate on the 

 changes that have occurred since the differentiation of organisms into animals 

 and plants. It is clear that the major steps in this great progression oc- 

 curred at a very early period, since the oldest fossil remains of animals, in 

 rocks of the Pre-Cambrian period (older than 500 million years), include 

 types representing all the great phyla except the Chordata. The conclusions 

 presented graphically by the family tree in Figure 7.3 are frankly specula- 

 tive; they are based on facts of structure and development observed in living 

 animals or are deduced from comparisons of fossil remains, interpreted in the 

 manner just described. 



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