THE CLASSIFICATION OF ANIMALS 



(Fig. 7.4) possess a body cavity termed a coelom, which is Uned by a layer ol 

 mesodermal tissue, the peritoneum. The manner of origin or mode of forma- 

 tion of the coelom differs in consistent ways in different kinds of eucoelomate 

 animals, and these facts are used in distinguishing between the two sub.series 

 which can be recognized. In contrast, members of the series Pseudocoelomata 

 have a body cavity which is not a coelom but a pseudocoel with no peritoneal 

 lining. 



Considering these broad, general characteristics, the phyla may be grouped 

 into larger subdivisions, according to a classification which seems reasonable 

 to many zoologists. It should be pointed out that these larger categories, the 

 branches, series, and so on, are not recognized taxonomic entities with the 

 same standing as, for example, the species, the family, or the phylum. They 

 are rather to be regarded as synthetic groupings, useful as aids to an under- 

 standing of the similarities and differences, and thus the probable ancestral 

 relationships, between the members of the various phyla. 



It should be realized also that any classification represents merely an opinion 

 of experts, based on the knowledge available at a given time. Aristotle, for 

 example, working in the fourth century B.C. with very little information 

 other than that which he himself could accumulate, classified animals as those 

 with red blood (essentially our vertebrates) and those without red blood (our 

 invertebrates). Early in the nineteenth century, the great French naturalist 

 Cuvier (1769-1832) divided the Animal Kingdom as it was known to him 

 into four main types: Vertebrate, Articulata, Mollusca, and Radiata (Zoophyta). 

 With increasing knowledge and increasing insight, it was later recognized that 

 each of these groups included forms so diverse that they could not properly 

 be classified together. Subsequent changes in Cuvier's scheme involved the 

 separation of the Articulata into the joint-footed Arthropoda and the worm- 



Fig. 7.5. Schematic cross sections for comparison of acoelomate, pseudocoelomate, and 

 eucoelomate plans of organization. A, acoelomate: epidermis and gastrodermis are separated 

 by mesenchymal "parenchyma," through which lymph-like fiuid percolates. B, pseudocoelomate: 

 the body wall encloses a cavity which contains muscular and other elements of mesodermal 

 origin, but the cavity is not lined, or the structures covered, by a peritoneal layer. C\ 

 eucoelomate: the body cavity is a true coelom, lined throughout by a continuous sheet of 

 mesodermal epithelium, the peritoneum; organs and structures that lie in the coelom are also 

 covered by the peritoneal layer and are suspended by double mesodermal sheets, the 

 mesenteries. In C, the peritoneum and mesenteries are represented by broken lines. 



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