CHAPTER 18 



Minor Phyla 



The major phyla, the ones composed of many and diverse kinds of 

 animals, are each discussed in separate chapters. The animal kingdom 

 contains in addition a number of forms which are not related closely 

 enough to any of these major phyla to be a class within one of them but 

 are classified as separate phyla. This emphasizes that a phylum is not a 

 large assemblage of organisms but a group of organisms which are so 

 unique in structure and function that they are not closely related to 

 any other group. Some of these minor phyla (Ctenophora, Nemertea, 

 Onychophora and Hemichordata) are treated elsewhere; the remaining 

 ones are described briefly here. 



1 57. Mesozoa 



The Mesozoa (Fig. 18.1 A) are minute parasites found in the body 

 cavities of certain invertebrates; one is found in the kidney of the 

 octopus. The body structure is the simplest of any multicellular animal. 



Anas 



Mouth- 

 Subgastric; 



Sta-Ik 



A 



Megozoa. 



Stomach 



B 



Figure 18.1. Mesozoa (A), showing the ciliated epithelium surrounding an inner 

 mass of reproductive cells, and Entoprocta (jB), showing some of the internal organs. 



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