. Elements of comparative anatomy. Anatomy, Comparative. FOEM OF THE BODY OF VEEMES. 131. Fig. 52. Young Taenia, with head pushed in. aHead. b Envelope, c The six embryonic hooks, remaining at one point of the envelope (after V. Siebold). Fig. 53. The same Taenia, with head protruded. Letters as in Fig. 52 (after V. Sic- bold). is based on the notion that certain abnormal external conditions of life gradually became normal, in consequence of the adaptation of the organism to them, and that it did not arise by a simple antagonism to the worm's primitive ontogenesis, which now includes this cyst
. Elements of comparative anatomy. Anatomy, Comparative. FOEM OF THE BODY OF VEEMES. 131. Fig. 52. Young Taenia, with head pushed in. aHead. b Envelope, c The six embryonic hooks, remaining at one point of the envelope (after V. Siebold). Fig. 53. The same Taenia, with head protruded. Letters as in Fig. 52 (after V. Sic- bold). is based on the notion that certain abnormal external conditions of life gradually became normal, in consequence of the adaptation of the organism to them, and that it did not arise by a simple antagonism to the worm's primitive ontogenesis, which now includes this cystic form as a normal part of its cycle. What has happened is this—that the process of adaptation has seized upon and exaggerated a normal inherited phase of the worm's ontogeny, and in virtue of the continuation of conditions favourable to the appearance of this exaggerated phase, its appearance has become a normal phenomenon. The variations of the cystic form are all readily deducible from the first developmental stage of the Cestoda. The embryo is generally provided with three pairs of hooks, and a cestoid head may be observed to be differentiated within it (Fig. 52, a); when fully developed this is pushed out, so that the envelope, which at first was external becomes the portion of the body below the head (Fig. 53, 6). In the Cysticercus-form the embryo grows into a vesicle filled with fluid, from the walls of which the head is budded out. When the head is protracted, the vesicle forms a terminal appendage of the body (Fig. 54). When a number of buds are formed on the wall of the vesicle, in which protractile heads are dif- ferentiated, we have the Coenurus-form. When the buds break off into the interior of the vesicle, and there form new vesicles, on the walls of which the same budding process goes on, leading to the for- mation of systems of vesicles, placed one within the other, and when the youngest of these can again bud off tasnia-heads on its inner wall, we get the E
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
