. Elementary biology, animal and human. Biology. ADDITIONAL ANIMAL STUDIES 177 comes in contact with the waving tentacles, the hydra ejects micro- scopic threads from certain cells (nettling cells) in the tentacles. The animal thus attacked is benumbed, and the hydra then uses the tentacles to push its prey into a mouth opening in the center of the circular row of tentacles. The food is drawn into the inside of the column, which is simply a hollow tube (Fig. 124). Here certain cells secrete digestive fer- ments which dissolve the foods that the animal has eaten, and the indi- gestible matter i
. Elementary biology, animal and human. Biology. ADDITIONAL ANIMAL STUDIES 177 comes in contact with the waving tentacles, the hydra ejects micro- scopic threads from certain cells (nettling cells) in the tentacles. The animal thus attacked is benumbed, and the hydra then uses the tentacles to push its prey into a mouth opening in the center of the circular row of tentacles. The food is drawn into the inside of the column, which is simply a hollow tube (Fig. 124). Here certain cells secrete digestive fer- ments which dissolve the foods that the animal has eaten, and the indi- gestible matter is ejected from the mouth. The digested food is then absorbed by the cells lining the cavity. Since the animal is bathed outside and inside by water contain- ing oxygen, t^e cells are able to absorb oxygen from the water and to give off carbon dioxid to the water. Hence no breathing organs are needed. It is evident that the tentacles with the netthng cells also serve to protect the hydra from too great familiarity on the part of visitors that might otherwise use it for food. When the hydra moves from one place to another, it bends over until the ends of the tentacles touch the surface on which it resrts. The tentacles then adhere to this. surface, the bottom of the column lets go, and the animal turns a somersault (Fig. 125) and lands on the lower part of the column; the process may then be again repeated. Like the higher animals the hydra reproduces by means of eggs and sperms. But it also has another interesting way of producing new individuals. On the surface of the column one frequently sees Uttle bunches. These are called buds (Fig. 124). They keep on growing outward tiU at last little tentacles and a mouth opening are. Fig. 125.—The movements made by hydra in locomotion. (Jen- nings.). 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 not perfec
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbiology, bookyear1912
