General Zoology

General Zoology

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1917 edition. Excerpt: ...males have a single tubular testis which opens through the anus. The eggs are fertilized far up in the ovarian tubules and pass down into the uteri where they have the shell added. C D E F G H I J K Fio. 77.--Life History of Ascaris. A, male; B, female; ex, excretory pore; u, female genital aperture; C, zygote; D, zygote without covering: E-H, cleavage stages; I, J, embryos; K, young Ascaris emerging from egg shell. A female Ascaris is continually laying eggs, producing as many as 15,000 in a day. A large number of reproductive products is commonly associated with parasitic life and is necessary because the chances of finding, a favorable place for development are small. This is, however, the only respect in which Ascaris shows adaptation to parasitism. The eggs pass out of the host and develop into larvae in moist earth (Fig. 77, C-K) but do not hatch unless they get into a man (K). They cannot infect until they have been upon the ground thirty to forty days, and then are introduced into the human body on polluted vegetables, in water, or from soiled hands. After gaining entrance the worm hatches, becomes mature, and begins to lay eggs in about a month. Recent investigations indicate that Ascaris may perhaps pass through larval stages in rats or mice before entering man. Comparing Ascaris with a flatworm, it shows two great structural advances--a body-cavity and an anus. The flatworm must ingest and egest through a single opening; the round-worm has a much more convenient arrangement with an entrance for food and an exit for faeces. The body cavity is not a true ccelom, like that in the earthworm (Figs. 16, 89) and other animals, but is sufficiently developed to make the general structure of Ascaris' body a tube-withina-tube, as described in...
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Product details

  • Paperback | 98 pages
  • 189 x 246 x 5mm | 191g
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236550870
  • 9781236550873