APPENDIX B 



THE ANIMAL KINGDOM 



In our treatment of the varied patterns of animal life (Chap. XIII) we 

 were not concerned with classification as such but merely with showing 

 how animals meet their common functional requirements by a variety 

 of structural patterns. It was sufficient for that purpose to group animals 

 under only four divisions — Protozoa, simple Metazoa at the cellular level, 

 intermediate Metazoa at the tissue level, and complex Metazoa at the 

 organ-system level of construction. This could be done only by ignoring 

 many important differences in structure. When these are taken into 

 consideration, it becomes necessary to recognize at least 11 animal phyla 

 of major importance: Protozoa, Porifera, Coelenterata, Ctenophora, 

 Platyhelminthes, Nemathelminthes, Mollusca, Echinodermata, Annelida, 

 Arthropoda, and Chordata. These 11 sharply distinguished phyla contain 

 by far the greater number of all animals. There are in addition a number 

 of lesser groups, some of which are clearly of phylum rank, others of 

 uncertain status; those that will require brief mention are the Nemertinea, 

 Rotifera, Bryozoa, Brachiopoda, and Onychophora. 



Characteristics used in classifying animals. The phyla are dis- 

 tinguished from one another not so much by the possession of unique 

 characteristics as by unique combinations of characters. Among the 

 important features in which they may differ are the following: 



1. Degree of Cell Differentiation. Animals are either (a) unicellular 

 (or composed of a relatively small number of undifferentiated cells) or 

 (b) composed of many cells exhibiting various degrees of differentiation 

 and division of labor. Those of the first category are the Protozoa; those 

 of the second, the Metazoa. 



2. Number of Germ Layers. Metazoa develop either two or three germ 

 layers (embryonic cell layers). Those with two germ layers are said to be 

 diploblastic; those with three germ layers are triploblastic. The three layers 

 are, beginning with the outermost, ectoderm, mesoderm, and entoderm. 

 Mesoderm is lacking in diploblastic animals. 



3. Kinds of Symmetry. Although a few animals are asymmetrical, 

 possessing no plane of symmetry, the great majority are built on a sym- 

 metrical plan, with spherical, radial, or bilateral arrangement of body 



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