. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 296 ANIMAL PHYSIOLOGY. tortuous or remain somewhat straight. The alimentary tract of a mammal passes through stages of development which cor- .â respond with the permanent form of other groups of verte- brates, according to a general law of evolution. Inasmuch as the embryonic gut is formed of mesoblast and hypoblast, it is easy to understand why the developed tract should so invaria- bly consist of glandular structures and muscular tissue dis- pos
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. 296 ANIMAL PHYSIOLOGY. tortuous or remain somewhat straight. The alimentary tract of a mammal passes through stages of development which cor- .â respond with the permanent form of other groups of verte- brates, according to a general law of evolution. Inasmuch as the embryonic gut is formed of mesoblast and hypoblast, it is easy to understand why the developed tract should so invaria- bly consist of glandular structures and muscular tissue dis- posed in a certain regular arrangement. The fact that all the. '"â " â tlrtin!'il\ll',il\tt""- Fig. 254.âPortion of a jelly-fish, the Medusa Aurelia^ showing gastro-vascular canals radi- ating from central stomach and terminating in a circular marginal canal (after Romanes). AU'these are shaded very dark ; the light spaces indicate artificial sections. Inasmuch as these canals as well as the stomach must contain some searwater, and since their contents represent the whole of the nutritive fluid (answering to the blood, lymph, and chyle of higher forms), we have both anatomically and physiologically a very crude or undifferen- tiated condition in such animals, and one of great interest from an evolutionary point of organs that pour digestive juices into the alimentary tract are outgrowths from it serves to explain why there should remain a physiological connection with an anatomical isolation. The general resemblance of the epithelium throughout, even in parts widely separated, also becomes clear, as well as many other points we can not now refer to in detail, to one who realizes the significance of the laws of descent (evolution). Comparative.^Amoeba ingests and digests apparently by every part of its body; though exact studies have shown that it neither accepts nor. retains without considerable power of. Please note that these images are extracted from sca
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