. The study of animal life. Zoology. 226 The Study of Animal Life part in ways from other Coelenterates, thus the characteristic stinging cells are modified into adhesive cells. The first and second series, separated by differences of structure and development, are yet parallel. In both there are polype-types ; in both medusoid types; in both there are single individuals and colonies of individuals; in both there are " ; We may compare a Hydra with a sea-anemone, a. medusoid with a jelly- fish, a hydroid colony with Dead-men's-fingers, Millepores with. Fig. 42.—The alternation
. The study of animal life. Zoology. 226 The Study of Animal Life part in ways from other Coelenterates, thus the characteristic stinging cells are modified into adhesive cells. The first and second series, separated by differences of structure and development, are yet parallel. In both there are polype-types ; in both medusoid types; in both there are single individuals and colonies of individuals; in both there are " ; We may compare a Hydra with a sea-anemone, a. medusoid with a jelly- fish, a hydroid colony with Dead-men's-fingers, Millepores with. Fig. 42.—The alternation "ol generations in the common jellyfish Aurelia. i, the free-swimming embryo; 2, the embryo settled down; 3, 4, 5, 6, the de- veloping asexual stages, or hydra-tubse; 7, 8, the formation of a pile of individuals by transverse budding; 9, the liberation of these individuals; 10, II, their progress towards the free-swimming sexual medusa form. (From the Evolution of Sex; after Haeckel.) the commoner reef-corals. Moreover, we may compare a medusoid liberated from a hydroid with Aurelia liberated from its fixed polype- stage, and permanently-free medusoids with jellyfishes like Pelagia. These are physiological parallels. The sedentary polypes are somewhat sluggish, with a tendency to bud and to form shells or skeletons of some kind. The free- swimming medusoid types are active, they rarely bud, they do not form skeletons, but their activity is sometimes expressed in. 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 Thomson, J. Arthur (John Arthur), 1861-1933. New York, C. Scribner's sons [printed at the Edinburgh press]
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1892
