Identifier: evolutionanimall00jord (find matches)
Title: Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals
Year: 1907 (1900s)
Authors: Jordan, David Starr, 1851-1931 Kellogg, Vernon L. (Vernon Lyman), 1867-1937
Subjects: Evolution
Publisher: New York, D. Appleton and Company
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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to thesurface of the water. Lungs are developing inside the body,and the tadpole is beginning to breathe as a land animal,although it still breathes partly by means of gills, that is, as GENERATION, SEX AND ONTOGENY 237 an aquatic animal. Soon it is apparent that although thetadpole is steadily and rapidly growing larger, its tail is grow-ing shorter and smaller instead of longer and larger. At thesame time, fore and hind legs bud out and rapidly take formand become functional. By the time that the tail gets veryshort indeed, the young toad is ready to leave the water andlive as a land animal. On land the toad lives, as we know,on insects and snails and worms. The metamorphosis of thetoad is not so striking as that of the butterfly, but if the tad-pole were inclosed in an unchanging opaque body wall whileit was losing its tail and getting its legs, and this wall wereto be shed after these changes were made, would not the meta-morphosis be nearly as extraordinary as in the case of the
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FIG. 140.—Metamorphosis of the toad: At left, strings of eggs; in water, various tad-pole or larval stages; and on the bank, the adult toads. (Partly after Gage.) butterfly? But in the metamorphosis of the toad we can seethe gradual and continuous character of the change. Many other animals, besides insects and frogs and toads,undergo metamorphosis. The just-hatched sea urchin doesnot resemble a fully developed sea urchin at all. It is a minute 238 EVOLUTION AND ANIMAL LIFE wormlike creature, provided with cilia or vibratile hairs, bymeans of which it swims freely about. It changes next intoa curious bootjack-shaped body called the pluteus stage. Inthe pluteus a skeleton of lime is formed, and the final true sea-urchin body begins to appear inside the pluteus, developingand growing by using up the body substance of the pluteus.Starfishes, which are closely related to sea urchins, show a similar metamorphosis,except that there is nopluteus stage, the truestarfish-shaped bodyforming w

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