File:Text-book of comparative anatomy (1898) (14778686192).jpg

Identifier: textbookofcompar01lang (find matches)
Title: Text-book of comparative anatomy
Year: 1898 (1890s)
Authors: Lang, Arnold, 1855-1914 Bernard, Henry Meyners Bernard, Matilda Haeckel, Ernst Heinrich Philipp August, 1834-1919
Subjects: Anatomy, Comparative
Publisher: London, New York, Macmillan and Co.
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Cnidaria the progressive development ofan intermediate layer between the outer body epithelium and theinner intestinal epithelium, this intermediate layer being formed of 104 COMPARATIVE ANATOMY CHAP. heterogeneous elements of independent origin, connective tissue, muscu-lature, nervous system, and sexual organs. The outer epithelium givesrise principally to the connective tissue and the nervous system, whilethe musculature and the sexual organs may be produced either by theouter or the inner epithelium. It is evident that the development of such an intermediate layer,which we call mesoderm, is the necessary preliminary of a higherorganological differentiation of the body. XII. Reproduction. Asexual reproduction by fission and gemmation—Stock formation—Division of labour and polymorphism. Asexual reproduction is very common among the Cnidaria side byside with sexual reproduction. Among the Ctenophora alone it has notbeen observed. In Hydra we find asexual reproduction by gemmation
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FIG. 78.—Bougainvillea ramosa (after Allxnan), with budding Medusa*, li, Nutritive polyps ;mk, medusa buds; m, detached young Medusae (Margelis ramosa). side by side with sexual reproduction in adult animals. Buds are formedby hollow outgrowths of the body wall. These buds grow, and at thedistal end a breach is formed—the oral aperture, round which thetentacles arise by means of new outgrowths. Such buds can detach II CNIDA RIA —REP ROD UCTION 105 themselves from the mother body, or they may in small numbersremain united with it for some time. In the last case small Hydracolonies composed of similar individuals arise. In the same way elegant and richly branched colonies arise inmost Hydroids (Fig. 78). The individuals of such stocks are, how-ever, generally not similar, but, as a consequence of more or lessdivision of labour, Dimorphism or Polymorphism takes place. Wedistinguish : (1) sterile nutritive persons, which remain on the level ofthe Hydroid, and undertake the feeding o

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