THE INVERTEBRATE PHYLA 111 



of them had an independent evolution from the Protozoa. Even 

 within the major groups such as the Arthropoda difficulties arise ; 

 it is becoming more certain that the Arthropoda are not a mono- 

 phyletic phylum of animals but instead are a grade of organisation 

 and that this grade has been reached independently many times 

 from some annelid-like stock (Tiegs and Manton 1958). 



It would appear that the relationship between the various 

 invertebrate phyla is a very tenuous one. There are many phyla 

 that seem to be isolated from each other, and even those phyla 

 that seem reasonably close to one another, on detailed examination 

 show differences as important as their similarities. Though it is 

 useful to consider that the relationships determined by com- 

 parative anatomy and embryology give proof of a monophyletic 

 origin of the major phyla, this can only be done by leaving out 

 much of the available information. Let us now consider the 

 invertebrate relationships determined by comparative biochemistry 

 and see if they lead to any more definite conclusions. 



