CHAPTER 12 



The Phyla Aschelminthes 

 and Nemertea 



All of the animals that remain to be considered have a body cavity, 

 or a circulatory system, or both. A circulatory system can be defined as a 

 system of channels containing a fluid that is moved around by muscular 

 activity. The walls of the channels are derived from mesoderm. Two 

 kinds of body cavities can be distinguished. Both are fluid-filled spaces 

 that permit the internal organs freedom of movement, unhampered 

 by extensive connection with the body wall. If the space lies between 

 the gastrodermis and tissues of mesodermal origin (i.e., if it surrounds 

 a gut made only of endoderm), it is a pseudocoelom. If the space lies 

 ivithin tissues of mesodermal origin (if it surrounds a gut composed 

 of gastrodermis covered with mesodermal tissues), it is a eucoelom or, 

 simply, coelom. A coelom is lined with a simple epithelium of meso- 

 dermal origin, the peritoneum. A pseudocoelom lacks an epithelium. 

 None of the pseudocoelomates has a circulatory system. 



The phylum Aschelminthes includes the pseudocoelomates whose 

 bodies are largely covered with cuticle. They have an anterior mouth 

 and a posterior anus. The phylum is large and includes groups of di- 

 verse appearance. 



The Nemertea are acoelomate (have no body cavity) but have a 

 circulatory system. The mouth is anterior and the anus posterior, and 

 in front of the mouth is an eversible proboscis. The phylum is small 

 and will be considered at the end of this chapter. 



Most of the remaining phyla, to be considered in later chapters, 

 have both a circulatory system and a eucoelom. 



81. Classification of the Aschelminthes 



The groups to be considered here have always been troublesome to 

 taxonomists. They have been arranged in one, two, three, and even six 

 different phyla. In the face of so many diverse opinions any one position 

 is necessarily arbitrary. It is largely for convenience, therefore, that the 

 groups will be treated as six classes in one phylum. The classes are 

 (Fig. 12.1): 



I. Rotifera. Aquatic microscopic animals with internal jaws and 



an anterior ciliated wheel-organ. 

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