Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 THE OEGANS. 35 one, or en do derm, which surrounds a primitive enteric cavity. The two layers pass into one another at the oral opening which leads into the cavity. The two cell-layers, which form the body of such an organism, furnish the conditions under which it is possible for it to lead an independent animal existence. The outer one is the organ of support, and may be converted into an organ of locomotion if it gives rise to cilia, and may be the seat of respiratory functions also.
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 THE OEGANS. 35 one, or en do derm, which surrounds a primitive enteric cavity. The two layers pass into one another at the oral opening which leads into the cavity. The two cell-layers, which form the body of such an organism, furnish the conditions under which it is possible for it to lead an independent animal existence. The outer one is the organ of support, and may be converted into an organ of locomotion if it gives rise to cilia, and may be the seat of respiratory functions also. In so far as it per- ceives the state of the surrounding medium it is an organ of sensation too. The inner layer is nutritive in function, produces changes in the food which is taken in, and allows what can be assimilated to pass into its cells; and these in their turn feed the outer layer of cells. What is useless is passed out again by the same opening as that by which it entered. As the functions of the two layers are dif- ferent the special characters of the morpho- logical elements which compose them are different also; we need only call attention now to the much greater size in most cases of the cells of the endoderm, as compared with the cells of the ectoderm. This grade of organisation is to be seen in some of the lower divisions of the Animal Kingdom (Coelenterata and Vermes), where it represents a lowly stage of development. Indications of it are to be seen even in the higher divisions. This form has been called the Gastrula, on account of the dominant development of the enteron. Starting from the hypothesis that forms agreeing with a Gastrula in all essential points were the precursors of all the higher forms of animal organisation, a Gastrasa-form resembling the Gastrula has been regarded as the primitive ancestral form of all animals. This Gastrasa theory is based, first, on the existence of independent animal forms which resemble the Gastrasa; secondly, on the fac
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