. Animal growth and development. Embryology; Growth; Biology; Growth; Embryology; Animals -- growth & development. The Development of o Primitive Animal 39 the sack (coelenteron) is elongated and narrow with its open end up. In the medusa, the sack is squat and rounded with its open end down. Some species appear both as polyps and medusae during their life cycles. In other species, one or the other adult form may be rudimentary and very short-lived or entirely absent. In both forms, the mouth is surrounded by tentacles, and these con- tain special stinging cells called cnidoblasts (hence t
. Animal growth and development. Embryology; Growth; Biology; Growth; Embryology; Animals -- growth & development. The Development of o Primitive Animal 39 the sack (coelenteron) is elongated and narrow with its open end up. In the medusa, the sack is squat and rounded with its open end down. Some species appear both as polyps and medusae during their life cycles. In other species, one or the other adult form may be rudimentary and very short-lived or entirely absent. In both forms, the mouth is surrounded by tentacles, and these con- tain special stinging cells called cnidoblasts (hence the name Cnidaria). Upon stimulation, they shoot out spear-like processes bearing powerful toxins to paralyze their prey (small crustaceans, insect larvae). Very little is known about the toxins. The few that have been studied appear to be proteins. When injected into rabbits, they produce neural damage and elicit the formation of antibodies, just as do certain snake venoms, tetanus toxin from bacteria, and other poisonous proteins. Life Cycle of the Genus Obefio Both polypoid and medusoid stages figure prominently in the life cycle of Obelia, which is summarized in Fig. 20. The polyps grow as colo- nies upon rocks and shells in shallow marine waters. The colony is fastened to the substratum by a mass of root-like runners that bear slender, branched stems, and from these extend polyps of two kinds: the feeding polyps equipped with mouth and tentacles, and the reproductive polyps, which lack all feeding mechanisms and are designed for one purpose—to produce medusae that bud off from the central cylinder and escape through the hole in the vase-shaped outer covering. Fig. 20. The life cycle of Obelia (after S+orer and Usinger). POLYP - MEDUSAE. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Sussman, Maurice.
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