GENERAL ZOOLOGY 



As we have seen, an individual phylum is made up of related classes con- 

 taining animals with similar fundamental characteristics. We assume that 

 similarity in structure implies common ancestry, and therefore that members 

 of the same phylum are more closely related to each other than to members 

 of other phyla. It is significant also that the various phyla can be grouped 

 into larger categories, on the basis of similarities and differences in large 

 general characteristics, as shown in Figure 7.4. A study of this figure will 

 demonstrate that just as a phylum can be subdivided into classes, orders, and 

 lesser ranks, the Animal Kingdom can be subdivided into subkingdoms, 

 branches, grades, series, and subseries. For purposes of grouping the phyla 

 into these larger categories, use is made of more general and fundamental 

 characteristics. These include the grade of organization, the kind of sym- 

 metry, and the presence or absence, and kinds, of body cavities. Since these 

 features are of such a basic nature and make possible such sweeping and gen- 

 eral distinctions within the Animal Kingdom, we may consider briefly what 

 they involve in terms of the structure of animals. 



^^ Grade of organization. This criterion distinguishes the kind of construc- 

 tion, whether unicellular as in the Protozoa, or multicellular, as in all other 

 animals. Among the multicellular forms, the Mesozoa and Parazoa are said 

 to represent the cellular grade of construction, with little or no difTerentiation 

 into tissues. All the higher animals, on the other hand, constituting the 

 Eumetazoa, possess groups of similar cells specialized for the performance of 

 particular functions and so are said to exemplify the tissue grade of 

 organization. 



^- Kind of symmetry. Among the Eumetazoa, two kinds of symmetry are 

 distinguishable. One, characteristic of the Coelenterata and Ctenophora, is 

 termed radial symmetry and is marked by the presence of a principal axis of 

 the body, about which the parts are disposed in a radiating pattern. In con- 

 trast to this, all the more complex metazoans possess bilateral symmetry, in 

 which there is one median or sagittal plane dividing the body into right and 

 left halves which are approximately mirror images of each other. There are 

 apparent correlations between forms of symmetry and ways of life in animals. 

 Radial symmetry is primarily associated with sessile or attached animals, 

 which are in approximately equivalent contact with their environment in all 

 directions; in radially symmetrical animals there are commonly diflerentia- 

 tions between oral and aboral ends, or between the free end and the attached 

 end, so that an axis of polarity usually corresponds to the axis of symmetry. 

 Bilateral symmetry, on the other hand, is generally associated with active, 

 free-moving ways of life, and in addition to right and left sides and head and 

 tail ends, back and belly surfaces may usually be distinguished. 

 ^- Body cavities (Fig. 7.5). Some of the bilateral animals have no cavities 

 within the body other than that of the digestive system; these are termed 

 acoelomate. Among the metazoans which do possess a body cavity surround- 

 ing the digestive tract, this cavity may have different characteristics and may 

 arise in different ways. For example, the members of the series Eucoelomata 



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