BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.2. Feeding mechanisms in animals

8.2.2. Tentacular feeding

The phylum Cnidaria comprises animals with a very simple body plan (sec. 2.8.2) — jellyfish, sea anemones, and the freshwater hydra. All these animals are carnivorous and use tentacles surrounding the Mouth to capture prey (see figs. 2.47 and 2.48). They lack a true gut; its function is served by a simple sac (the gastrovascular cavity) with a single opening through the mouth. Food enters the Organism via the mouth (ingestion), and all indigestible waste is likewise expelled through it.

Stinging Cells known as nematoblasts are distributed along the outer surface of the tentacles. When stimulated, a Hair-like trigger mechanism activates, causing an explosive discharge of The Cell contents (fig. 8.6). Typically, two simultaneous stimuli are required to trigger these cells, such as physical Touch and specific chemical cues (the "taste" or "smell" of prey). There are several types of nematoblasts. Some bear barbs that pierce the prey during the "attack" phase. Others possess small hooks that snag the prey, alongside long hollow threads (such as the stinging threads of jellyfish) that release venom capable of paralyzing or even killing the victim. These threads may also be sticky and serve to entangle prey. Once immobilized and ensnared, the prey is transferred by the tentacles into the mouth for Digestion. Through the widely opened mouth, the prey enters the gastrovascular cavity, initiating the initial extracellular phase of digestion. Once the food is broken down into smaller fragments, it is engulfed by phagocytosis by the cells lining the gastrovascular cavity, where intracellular digestion is then completed. The prey of Hydra typically consists of Daphnia and Cyclops, small freshwater crustaceans.

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Fig. 8.6. A. Hydra capturing a Water flea (Daphnia). B. A nematoblast before and after discharge. C. A group of nematocysts from the sea anemone Rhodactis rhodostoma. Except for two, all nematocysts have already discharged their stinging threads. Light micrograph. D. Trichodina captured by Hydra tentacles. Scanning electron micrograph.