Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 ill COMPARATIVE ANATOMY. of metameres that have been suppressed iu the course of this differentiation. Further differentiations are brought about by the development of paired appendages. The hinder ones in the Gna- thostomata separate the trunk and tail more distinctly from one another, as do the anterior ones in the case of the head and trunk. When the anterior appendages are separated from the head, as they are even in the Selachii among the Fishes, a cervical region is differentiated fr
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 ill COMPARATIVE ANATOMY. of metameres that have been suppressed iu the course of this differentiation. Further differentiations are brought about by the development of paired appendages. The hinder ones in the Gna- thostomata separate the trunk and tail more distinctly from one another, as do the anterior ones in the case of the head and trunk. When the anterior appendages are separated from the head, as they are even in the Selachii among the Fishes, a cervical region is differentiated from the trunk, and connects it with the head. We meet with this arrangement in the Amphibia. The trunk is affected by further differentiations, and in the Amniota is separated into cervical, thoracic, and lumbar regions. The caudal portion of the body undergoes a gradual metamor- phosis. In the Fishes it is scarcely marked off; in the Amphibia (Urodela) and Reptilia (Saurii, Crocodilini) it is separated from the trunk by the hinder pair of appendages, and by its diminished bulk. Although it is atrophied in Birds, it is in the Mammals only that it acquires the character of an appendage of the body, in consequence of its great decrease in bulk, even though it may still be of some length. Appendages. § 317. We must divide the appendages which are given off from the body of the Vertebrata, and which function principally as locomotor organs, into paired and unpaired parts. The unpaired ones are developed from a vertical membrane—a continuation of the integu- ment—which extends from the head to the anus. When firm structures and special muscles are developed in this membrane, this purely dermal process is converted into a fin. This organ either retains its primitive continuity of arrangement (Fig. 214, A), or by the atrophy of some, and by the in- creased development of the remaining portions, is broken up into several parts. These are dis- tinguished according to their position
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