CHAPTER 



12 



SOME PSEUDOCOELOMATE 



ANIMALS AND MINOR 



EUCOELOMATE PHYLA 



The animals considered in this chapter are structurally much more ad- 

 vanced than the flatworms. Perhaps the most significant feature of their 

 particular level of organization is the presence of a body cavity between 

 the gut wall and the body wall. Some kind of body cavity is present in all 

 the higher animals, although in different forms it is variously developed and 

 arises in difi"erent ways (pp. 222-223). In one small group of phyla the 

 body cavity represents a development of the embryonic blastocoel and is not 

 lined by a mesodermal peritoneum. Such a cavity is termed a pseudocoel, and 

 animals possessing this kind of a body cavity may be grouped together as the 

 series Pseudocoelomata (Fig. 7.4, p. 221). Of the three phyla constituting 

 this series, we shall consider in detail only the phylum Aschelminthes, re- 

 ferring more briefly to the phyla Acanthocephala and Entoprocta. 



The other phyla to be discussed in this chapter are of a different grade of 

 construction. They have a body cavity which is a true coelom, and their 

 affinities lie with the more advanced major phyla which will be considered in 

 subsequent chapters. The phyla Ectoprocta, Brachiopoda, and Chaetognatha 

 are called minor phyla because they consist of relatively small numbers of 

 species. They are discussed together in this chapter not because they form 

 a coherent, interrelated group, but because it is impossible to state with 

 any certainty where they should be placed, in relation to the larger groups, 

 in the phylogenetic scheme of the Metazoa. 



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