. An introduction to zoology [microform] : for the use of high schools. Zoology; Zoologie. Fig. 176.âThread-cells of Hydroid. I, undergoing development; II, with the thread protruded. 5. The Coelenterata also are "radiate," but the parts are usually disposed in twos, or fours, or sixes ; instead of the complete intestine and complicated c(jelom of the preceding group, there is only one cavity which discharges the functions of both; peculiar modified cells (thread-cells or nettling organs, Fig. 176), take on a defensive function, by pouring an acrid poison into wounds inflicted by the
. An introduction to zoology [microform] : for the use of high schools. Zoology; Zoologie. Fig. 176.âThread-cells of Hydroid. I, undergoing development; II, with the thread protruded. 5. The Coelenterata also are "radiate," but the parts are usually disposed in twos, or fours, or sixes ; instead of the complete intestine and complicated c(jelom of the preceding group, there is only one cavity which discharges the functions of both; peculiar modified cells (thread-cells or nettling organs, Fig. 176), take on a defensive function, by pouring an acrid poison into wounds inflicted by their microscopic barbs. From the tendency to form colonies (often plant-like in form) this group, which is almost exchisivelv marine, used to be called the Zoophytes. Apart from some singular pelagic forms which have comb-like ridges of cilia on the surface (Ctenophora), the Ccelenterates fall into two classesâthe Hydrozoa and the Actinozoa. The former name is derived from the fresh-water Polyp, Hydra (Fig. 177), a cosmopolitan form very much simpler in its organi- zation than its marine allies. It does not form colonies, but buds off individuals which soon become detached from the parent. Eggs are formed, during a short period of the year, in the wall of the two-layered tube which constitutes the body. Fig. 177.âHydra viridit, at- tached to a buds in two stages of development. The expanded tentaclesappear granular owing to accumula- tions of thread-cells, a, a thread-cell. â rt ' ! t 31: " f 111. 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 Wright, R. Ramsay (Robert Ramsay), 1852-1933. Toronto : Copp, Clark
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishe, booksubjectzoology
