. Biology; the story of living things. DIVISION OF LABOR IN THE COELENTERATES IJil tain chemical stimuli that apparently cause a change of osmotic pres- sure within the cell, thus forcing out its threadlike portion. After a nematocyst is protruded, the cnidoblast dies and is soon replaced by another. stinSina. "nerve cell "muscular absorbing cell ^ .flagellum -sensory cell -^ cell ® cxxnthmd cell The Endoderm and Its Functions By cutting a section through the body of a Hydra its similarity to a two-walled sac is evident. Between the ectoderm and the inner layer of cells (endoderni) a
. Biology; the story of living things. DIVISION OF LABOR IN THE COELENTERATES IJil tain chemical stimuli that apparently cause a change of osmotic pres- sure within the cell, thus forcing out its threadlike portion. After a nematocyst is protruded, the cnidoblast dies and is soon replaced by another. stinSina. "nerve cell "muscular absorbing cell ^ .flagellum -sensory cell -^ cell ® cxxnthmd cell The Endoderm and Its Functions By cutting a section through the body of a Hydra its similarity to a two-walled sac is evident. Between the ectoderm and the inner layer of cells (endoderni) a thin, structureless layer called the mesoglea forms as a secretion from the cells of the «^toclerm j e«dod^m inner and outer layers. Mesoglea forms much of the bulk of other coelenterates like the jellyfishes. The endo- derm consists principally of large vacuolated cells that have flagella at the free or inner end, al- though they are also capable of developing pseudopodia at this end. Circular contrac- tile fibers are developed at their basal end. Thus they are endothelial-muscular cells. In the third of the body nearest the basal end, gland cells develop, which secrete digestive enzymes. Nerve and sensory cells are also found in the endoderm. For a simple animal, the Hydra seems to have many kinds of cells. What is the use of so many ? The answer is found in the way it gets food, ingests it, and finally absorbs it into the body cells. By watch- ing a hydra in the aquarium it will be seen that its tentacles are con- stantly moving as if seeking food. If a tiny bit of raw beef is placed within reach, the animal will bend over and carry the meat to the mouth, the edges of which soon close around it, forcing it inside. If the piece is too large to be taken in, the Hydra actually turns inside out in an attempt, usually successful, to put the meat inside the gastrovascidar cavity. Once inside the cavity, digestive enzymes from the glandular cells act upon the food, gradually br
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