. A manual of zoology. Zoology. CCELENTERATA. 229 cavity this is seen in development to arise as diverticula from the primitive stomach (enterou). Since such diverticula occur in coalenteratos without becoming independent, one can say that the gastrovascular system consists not only of intestinal portions but, in potentia, of the crelom as well. To even a superficial observation the Crelenterata are more clearly animals than are the sponges. The single animals, though often united in colonies, and fixed to some support, are capable of quick and energetic motion. These movements are most striki
. A manual of zoology. Zoology. CCELENTERATA. 229 cavity this is seen in development to arise as diverticula from the primitive stomach (enterou). Since such diverticula occur in coalenteratos without becoming independent, one can say that the gastrovascular system consists not only of intestinal portions but, in potentia, of the crelom as well. To even a superficial observation the Crelenterata are more clearly animals than are the sponges. The single animals, though often united in colonies, and fixed to some support, are capable of quick and energetic motion. These movements are most striking in the tentacles—long tactile threads, in the neighborhood of the mouth, which have the functions of feeling for food, grasping it, and conveying it to the mouth. The means of killing the prey are the cnidse, nematocysts, or nettle cells (fig. 171), which with rare. Fig. 171.—Nettle cells of Coelenterata, (After Hertwi^. Lendenfeld, and Hamaiin.) exceptions in Protozoa, Turbellaria, and molluscs occur in no other group. These structures, of great systematic importance, are oval vesicles with fluid contents and firm membrane. Each is drawn out at one end into a long tube, so delicate as to appear as a thread (hence an additional name, thread cells). This thread is sometimes armed throughout its length with retrorse hooks, or it may have only a few stronger hooks on its basal portion, whicli is thicker than the rest. In the resting stage the thread is spirally coiled inside the cell. On stimulation the thread is quickly extended ('explosion of cell') and i)roduces a wound into which passes the irritating fluid contents. Some cffilenterates { PJiysalia) can produce in this way very painful nettling even in man. The nettle cajisulc arises as a plasma product inside a cell. When fully developed the nettle cell extends to the surface and ends with a tactile jirocess (cnidocill) which upon contact stimu- lates the protoplasm and causes the explosion. The cell itself is freq
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