. Biology; the story of living things. DIVISION OF LABOR IN THE COELENTERATES 183 and the foot. This network of cells lies in the ectodermal layer of the animal, and receives impulses from sensory cells as well as trans- mitting them to the muscle fibrils. The sensory cells of the ectoderm .vary in their location; one type occurs on the tentacles, one on the hypostome, and a third on the foot (base). Neuro-sensory cells which are located in the mid-body area .also resemble nerve cells, except that they send processes to muscle fibrils and so become intermediate between those receiving stimulat
. Biology; the story of living things. DIVISION OF LABOR IN THE COELENTERATES 183 and the foot. This network of cells lies in the ectodermal layer of the animal, and receives impulses from sensory cells as well as trans- mitting them to the muscle fibrils. The sensory cells of the ectoderm .vary in their location; one type occurs on the tentacles, one on the hypostome, and a third on the foot (base). Neuro-sensory cells which are located in the mid-body area .also resemble nerve cells, except that they send processes to muscle fibrils and so become intermediate between those receiving stimulation and those making the response. Some nerve cells appear in the endodermal layer but are not, so far as can be deter- mined, connected with the ecto- dermal nerve net. Reproduction Probably the most important function of the interstitial cells is their growth into sex cells. Most Hydras are hermaphroditic, that is, have both kinds of sex cells present in the same individual, but since the sperm cells and ova ripen at different times, fertilization is accomplished by sex cells from different indi- viduals. Sperm cells are produced by the mitotic division of interstitial cells, each of which first produces a number of parent male cells, contain- ing the somatic number of The nerve net in a young hydra as seen with an intravitani methylen- blue stain. Note the ringlike ar- rangement in hypostome and foot. What effect might such an arrange- ment have on movement? (After J. Ilodzi.) These cells divide four times and in the process a reduction division takes place, leaving the sperm cells with just half as many chromosomes as the body cells. A somewhat similar process takes place in the formation of the ova. One interstitial cell becomes larger than the others, rounds into a sphere, and is surrounded by other interstitial cells, which serve as an ovary for the growing egg. The latter continues to grow in size, form- ing yolk from the surrounding cells. Just before the
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