248 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



with modification from some primal form of mollusk; 

 and radiates (including echinoderms and ccelenterates) 

 from some primal form of radiate animal. 



That these prime divisions also have a common 

 origin is shown by their common cellular structure, and 



also by the fact that the first 

 steps in embryonic devel- 

 opment is the same in all. 

 But this common origin is 

 not shown in the completed 

 structure. 



From this point of view, 

 at least four main branches 

 of the tree of life came from 

 the protozoan trunk and grew 

 in different directions, di- 

 verging more and more, and 

 giving off each subordinate 

 branches (classes, orders, families, etc.) ; but the point of 

 union with the protozoan trunk and with one another is, 

 as it were, hidden from view beneath the common ground 

 of cellular structure. But the subordinate branches of 

 each primary branch can be traced throughout. Dia- 

 gram Fig. 158 roughly expresses what we mean. 



Fig. 158. — Diagram showing sup- 

 posed common origin of the dif- 

 ferent phyla. 



SECTION II. 



Homology of Vertebrates. 



The thesis to be established is that all vertebrates 

 have come from some primal form of vertebrate by 

 modification. This is best shown in the skeleton, and 

 we shall confine ourselves mainly, though not entirely, 

 to this system. The common origin is shown, first, 

 in the 



