. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 154 ELEMENTS OF BIOLOGY some, especially those which feed on plants, require only compara- tively simple apparatus for dislodging and breaking off their food substances; for example, cattle. In general, animal foods are in solid form, or at least in chemical forms that require chemical change before the materials can be utilized by protoplasm or the energy in the food released. The conversion of this r^LW material into chemical and physical conditions that make it available to the cells of th
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 154 ELEMENTS OF BIOLOGY some, especially those which feed on plants, require only compara- tively simple apparatus for dislodging and breaking off their food substances; for example, cattle. In general, animal foods are in solid form, or at least in chemical forms that require chemical change before the materials can be utilized by protoplasm or the energy in the food released. The conversion of this r^LW material into chemical and physical conditions that make it available to the cells of the animal body comprises the whole process of digestion. The details of the process difiFer in different forms; however, all animal digestive systems are designed to accomplish this common COOP GIZZARD TYPHLOSOLE PHAPYNX WAU. MOUTH PHAPYNX OESOPHAGUS INTESTINE ANUS Fig. 101.—Digestive system of the earthworm. result. They involve (i) machinery for obtaining and mechanically breaking up the food; (2) enzymes and other chemical agents for dissolving and altering the food chemically; (3) areas for the ab- sorption of the finished products of digestion; (4) a mechanism for conveying these products from the site of digestion to all cells of the body. This fourth principle links the digestive system with the circulatory system and will be considered farther on. Abrading Mechanisms. The first of these, mechanically breaking up the food, is accomplished by various devices usually, but not invariably, associated with that part of the digestive system which receives the food. In the diploblastic Porifera and Coelen- terata and in Platyhelminthes, a considerable proportion of the digestive process is accomplished within the endodermal cells, that is, intracellular, and no mechanisms are provided for grinding or. 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
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