![]() Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). The microscopic threads then either entangle the prey or instantly penetrate the flesh of the prey or predator, releasing toxins (including neurotoxins and pore-forming toxins that can lead to cell lysis) into the target, thereby immobilizing it or paralyzing it (see (Figure)).Ĭnidarian body forms. If the cnidocils are touched, the hollow threads evert with enormous acceleration, approaching 40,000 times that of gravity. The outer wall of the cell has a hairlike projection called a cnidocil, which is sensitive to tactile stimulation. Cnidocytes have large stinging organelles called nematocysts, which usually contain barbs at the base of a long coiled thread. These cells are located around the mouth and on the tentacles, and serve to capture prey or repel predators. Whereas the defining cell type for the sponges is the choanocyte, the defining cell type for the cnidarians is the cnidocyte, or stinging cell. Nearly all (about 99 percent) cnidarians are marine species. Phylum Cnidaria includes animals that exhibit radial or biradial symmetry and are diploblastic, meaning that they develop from two embryonic layers, ectoderm and endoderm.
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