![]() Some cnidarians are dimorphic, that is, they exhibit both body plans during their life cycle. ![]() 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)). 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. Describe the identifying features of the major cnidarian classes.Identify the two general body forms found in the Cnidaria.Describe the progressive development of tissues and their relevance to animal complexity.Compare structural and organization characteristics of Porifera and Cnidaria.By the end of this section, you will be able to do the following:
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