CHAPTER 7 



THE INVERTEBRATE PHYLA 



Within the invertebrates there are many distinct phyla. So far 

 we have considered some of the possible relationships between the 

 so-called " lower phyla," namely the Protozoa, Porifera, Mesozoa, 

 Coelenterata, Ctenophora and Platyhelminthia. There are, how- 

 ever, many other important phyla such as the Nematoda, Nemertea, 

 Rotifera, Annelida, Arthropoda, Mollusca, Brachiopoda, Echino- 

 dermata and Protochordata that all deserve some mention for they 

 each present special problems of phylogenetic relationship. 



It is not possible to obtain satisfactory palaeontological data 

 concerning the relationship of these various phyla because most 

 of them are already fully established in the earliest fossil-bearing 

 beds, the Cambrian. This means that one has to use other inform- 

 ation to determine the relationships between these phyla. In 

 fact these relationships are not at all clear and this can best be 

 illustrated by examining three attempts that have been made to 

 present a coherent monophyletic relationship of the major 

 invertebrate phyla. 



Grobben's Classification 



Karl Grobben in 190S proposed a scheme to show the inter- 

 relationship of various invertebrate groups. This system has 

 formed the basis for most of our current schemes, e.g. Cuenot 

 1952. The system divides the major invertebrate phyla into two 

 sections, the Protostomia and the Deuterostomia. This distinction 

 had been proposed by Goette in 1902 and was based on the fate 

 of the blastopore in the developing embryo: whether it becomes 

 the anus or the mouth and anus. In the Protostomia the blastopore 

 becomes the mouth and anus whilst in the Deuterostomia it 



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