Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 METAZOA. All remaining divisions or groups of animals are united under the name Metazoa for the following reasons: A careful consideration of their structure leads to the con- clusion that in all the body is of appreciable size, and that in each and every one certain portions or organs are specialized for the performance of certain functions neces- sary in the economy of the individual. Thus we find in all reproductive organs which have solely to do with the perpetuation of the species;
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 METAZOA. All remaining divisions or groups of animals are united under the name Metazoa for the following reasons: A careful consideration of their structure leads to the con- clusion that in all the body is of appreciable size, and that in each and every one certain portions or organs are specialized for the performance of certain functions neces- sary in the economy of the individual. Thus we find in all reproductive organs which have solely to do with the perpetuation of the species; in all (except a few degener- ate parasites) there is an opening (mouth) for taking in of food and an alimentary tract for its digestion; in all there is a more or less distinct ner- vous system; and in all parts of the body are more or less specialized for respiration. A little deeper insight leads to another conclusion which further justi- fies the group of Metazoa. The body is composed of layers, at least two in number, one on the outside forming the skin, and a second on the inside forming the lining of the digestive tract. To these two layers are fiven names, ectoderm and entodcrm (fig. 9), meaning respectively outer and inner skin. 154 ec FIG. 9.—Diagram of a two-layered animal, based upon a hy- droid. ec, ectoderm; en, entoderm.
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