First lines of physiology, being an introduction to the science of life; written in popular language . t appear that any particularorgan of taste or smell is required to enable them todistinguish what food is proper or injurious for them,but they take what the beneficence of Providence sendsthem, without asking questions. They do not chewtheir food, and hence require no solid parts like teeth orjaws. We shall soon perceive, however, that muchmore complex apparatus is employed by animals a Httlemore elevated in the scale of creation. 122. In many of the medusae (52) the thickness andbulk of the
First lines of physiology, being an introduction to the science of life; written in popular language . t appear that any particularorgan of taste or smell is required to enable them todistinguish what food is proper or injurious for them,but they take what the beneficence of Providence sendsthem, without asking questions. They do not chewtheir food, and hence require no solid parts like teeth orjaws. We shall soon perceive, however, that muchmore complex apparatus is employed by animals a Httlemore elevated in the scale of creation. 122. In many of the medusae (52) the thickness andbulk of the body or cap of the animal is so great thatit cannot be conveniently nourished by absorption froma simple central cavity ; and in these animals we findthe .cavity which answers the purpose of a stomachdivided into four principal sacs in the forai of a cross,the corners of which are extended into tubes that pene-trate the substance of the body, ramifying continuallyas they go, the smaller branches opening into eachother, so as to form at last a complete net-work of canals, 72 MASTICATORS through which the sea-water, to- Fig. 22. gether with the digested food which it contains, may be driven about from place to place for the support of all parts of the frame. In fig. 22 you see a portion of the edge of the medusa represented at fig. 2. The irregular white lines represent the ramifications of the stomach. Edge of Medusa. 123. You have been told that the central cavity ofthe hydra (60) seems to fulfil the double purpose of aheart and a stomach; but in the medusa this is muchmore obvious. 124. In all the simple animals of which we have beenspeaking, the functions of digestion (58) and assimilation(47) appear to require no complex apparatus; for theirfood is taken into the stomach without previous prepara-tion, and with very little, if any, selection, and the ordi-nary contractility of cellular tissue is sufficient to effectall the slow and gentle motions which are requir
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