Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 16 PAPiT. organ (kidney), mostly in a fluid form. For the movement and circulation of the fluid carrying the absorbed nutriment, there is a pulsatory pump (heart) and a system of blood vessels, while respira- tion is usually carried on in terrestrial animals by lungs, and in aquatic animals by gills. Finally, animals possess internally placed generative organs, and a nervous system, and sense organs for the production of sensation. In plants, on the con
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 16 PAPiT. organ (kidney), mostly in a fluid form. For the movement and circulation of the fluid carrying the absorbed nutriment, there is a pulsatory pump (heart) and a system of blood vessels, while respira- tion is usually carried on in terrestrial animals by lungs, and in aquatic animals by gills. Finally, animals possess internally placed generative organs, and a nervous system, and sense organs for the production of sensation. In plants, on the contrary, the vegetative organs have a much simpler form. Roots serve to absorb fluid nutriment, while the leaves act as respiratory and assimilating organs, taking in and giv- ing out gas. The complicated systems of organs found in animals are absent, and a more uniform parenchyma of cells and vessels, in which the sap moves, composes the body of plants. The gener- ative organs also are placed in external appendages, and there are no nervous and sense organs. Nevertheless, the above mentioned differences are not universally found, but rather hold only for the higher animals and plants, and gradually disappear with the simplification of the organization. Even among vertebrates, and still more is it the case amongst mollusca, and the lower segmented animals, the respiratory and vascular organs are considerably simplified. The lungs or gills may fail as special organs, and be replaced by the whole outer surface of the body. The blood vessels are simplified, and sometimes they and the heart are absent, the blood being moved in more irregular streams in the body cavity and in the wall-less spaces in the organs. Similarly, the digestive organs are simplified; salivary glands and liver may no longer be found as glandular appen- dages of the alimentary canal. The alimentary canal may become a blind, branched, or simple sac (Trematoda), or a central cavity, the walls of which are in conta
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