. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. CGELENTERATES 199 Draw the medusa twice natural size, showing all the above parts neatly labeled. Development. — The egg of the medusa after fertilization under- goes a number of changes. First the egg splits in two, then four, eight, and ultimately a mass of cells. This process is known as segmentation. These cells form a hollow ball of cells and swim through the water by means of cilia. Ultimately this little animal settles down on one end and becomes fixed to a rock, seaweed, or pile. Th
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. CGELENTERATES 199 Draw the medusa twice natural size, showing all the above parts neatly labeled. Development. — The egg of the medusa after fertilization under- goes a number of changes. First the egg splits in two, then four, eight, and ultimately a mass of cells. This process is known as segmentation. These cells form a hollow ball of cells and swim through the water by means of cilia. Ultimately this little animal settles down on one end and becomes fixed to a rock, seaweed, or pile. The free end becomes indented in the same manner as a hollow rubber ball may be pushed in on one side. This indented side becomes a mouth, tentacles develop around the orifice, and we have an animal that looks very much like the hydra. This animal, now known as a hydroid polyp, buds rapidly and soon forms a colony of little polyps, each of which is connected with its neighbor by a hollow food tube. The hydroid polyp differs from its fresh-water cousin, the hydra, by usually possessing a tough covering which is not alive. Hydroid Colony {Pennaria)} — (Material put up in formol may be handed out to the class in small vials). In the portion of the colony you have, where are the polyps located ? Examine a single polyp and make out all you can regarding (a) its general form, (6) the position of tentacles, (c) the position of the mouth. The buds which form the free-swimming medusae are frequently found bud- ding out of the wall of the polyp. Can you de- scribe them? Demonstration. — Stained polyps and part of the branch, using the compound microscope. Draw part of a colony four times natural size. Label all the points mentioned above. Alternation of Generations in Coelen- terates. — The lives of a hydroid and a medusa are seen thus to be intimately con- nected with each other. A hydroid colony produces new polyps by budding. This we know is an asexual method of repro- duction.
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