Page:A Text-book of Animal Physiology.djvu/56

 if we exclude the doubtful case of the thread-cells (urticating capsules). The animal breathes by the entire surface of the body; nourishment passes from cell to cell, and waste is discharged into the water surrounding the creature from all cells, though probably not quite equally. All parts are not digestive, respiratory etc., to the same degree, and herein does it differ greatly from Amœba or even Vorticella, though fuller knowledge will likely modify our views of the latter two and similar organisms in this regard.

THE CELL RECONSIDERED. Having now studied certain one-celled plants and animals, and some very simple combinations of cells (molds, etc.), it will be profitable to endeavor to generalize the lessons these humble organisms convey; for, as will be constantly seen in the study of the higher forms of life of which this work proposes to treat principally, the same laws operate as in the lowliest living creatures. The most complex organism is made up of tissues, which are but cells and their products, as houses are made of bricks, mortar, wood, and a few other materials, however large or elaborate.

The student of physiology who proceeds scientifically must endeavor, in investigating the functions of each organ, to learn the exact behavior of each cell as determined by its own inherent tendencies, and modified by the action of neighboring cells. The reason why the function of one organ differs from that of another is that its cells have departed in a special direction from those properties common to all cells, or have become functionally differentiated. But such a statement has no meaning unless it be well understood that cells have certain properties in common. This is one of the lessons imparted by the preceding studies which we now review. Briefly stated in language now extensively used in works on biology, the common properties of cells (protoplasm), whether animal or vegetable, whether constituting in themselves entire animals or plants, or forming the elements of tissues, are these: The collective chemical processes associated with the vital activities of cells are termed its metabolism. Metabolism is constructive when more complex compounds are formed from simpler ones, as when the Protococcus-cell builds up its protoplasm out of the simple materials, found in rain-water, which make up its food. Metabolism is