4 Comparative Animal Physiology 



recent years many physiological characters have been examined with respect 

 to their evolutionary significance. Physiological and biochemical homologies 

 and analogies can be established and are as striking as morphological ones. 

 Evidences of embryonic retention, of convergence and divergence, of loss of 

 function without return have also been presented for biochemical characters, 



CEPHALOPODA 



CEPHALOCHORDA 



UROGHORDA 



ECHIUROIDEA 



TROCHOPHORE TYPE 



MERTINA 

 PLATYHELMINTHES 



ASCHELMINTHES 



PRIMITIVE ACOEL FLATWORMS 



CTENOPHORA 



CNIDARIA 

 (COELENTERATA) 



PRIMITIVE MEDUSAE 



PLANULA TYPE ' 



OTHER PROTOZOA 



FLAGELLATA 



Fis- 1- Phvloacnetic tree modified from Hyman.' 



particularly in embryos. It is important to learn how far functional evidence 

 agrees with the traditional morphological evidence for animal kinships. 



To provide a reference point for phylogenetic discussions a brief summary 

 of present views regarding the history of the principal animal phyla is neces- 

 sary. A "tree" of the animal kingdom is given in Figure 1. There is good 

 agreement that at the level of the lower worms there occurred a divergence 

 which resulted in two important parallel lines. On the one side there arc 

 the annelids, arthropods, and molluscs; on the other side are the echinoderms 



