. The chordates. Chordata. Fig. 8. Narwhal skull. Bone has been removed to expose the root of the tusk and the rudimentary tusk of the other side. (Courtesy, Flower and Lydekker: ''Introduction to the Study of Mammals," London, A. & C. Black, Ltd.). Fig. 9. Flounder, Paralichthys oblongus (Mitchill). (Courtesy, Jordan and Evermann: " Fishes of North and Middle America," Washington, The Smithsonian Institution.) the development of the adult there occur numerous and various devia- tions from this plan, either where symmetry is not necessary for func- tion, or where departure f
. The chordates. Chordata. Fig. 8. Narwhal skull. Bone has been removed to expose the root of the tusk and the rudimentary tusk of the other side. (Courtesy, Flower and Lydekker: ''Introduction to the Study of Mammals," London, A. & C. Black, Ltd.). Fig. 9. Flounder, Paralichthys oblongus (Mitchill). (Courtesy, Jordan and Evermann: " Fishes of North and Middle America," Washington, The Smithsonian Institution.) the development of the adult there occur numerous and various devia- tions from this plan, either where symmetry is not necessary for func- tion, or where departure from symmetry favors function, as in the re- productive organs of female birds. Head, Neck, Trunk, Tail In the external form of a vertebrate, these four regions are more or less definitely demarked. A fish has no obvious neck. But the gills and certain glands (thyroid and thymus), situated just at the rear of the head, mark a region corresponding to the anterior region of the neck of a mammal. In contrast to fishes are the swan, ostrich, and giraffe. The elongated flexible muscular neck of birds and mammals gives the advantage of free mobility of the head. A fish, to look behind, must turn the whole body. The trunk is hollow, and its chief cavity (body-cavity or coelom) contains the various bulky viscera of the animal (Fig. 10). The coelom does not extend into the neck or tail of the adult animal. The tail is the solid muscular extension of the animal behind the hollow trunk. In most fishes the trunk tapers gradually into the nar-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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