File:Image from page 183 of "Introduction to zoology; a guide to the study of animals, for the use of secondary schools;" (1900) (14784662562).jpg

Identifier: introductiontozo00dave Title: Introduction to zoology; a guide to the study of animals, for the use of secondary schools; Year: 1900 (1900s) Authors: Davenport, Charles Benedict, 1866-1944 Davenport, Gertrude Anna Crotty, 1866- Subjects: Zoology Publisher: New York, Macmillan company London, Macmillian and co., ltd. Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library

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Text Appearing Before Image: rawn tovarious scales. A, Haminea; /:>, Sca-phander; C, Aplustrum ; I), Aphysia ;E, Philine. From Cooke, Mollusca. the shell is reduced to afew calcareous grains.An exactly similar seriesin the degeneration of the shell is found in certain Opistho-branchs, illustrated in Fig. 149. The beginnings of thisprocess of covering the shell are seen in many gastropodsin which the mantle edges may protrude beyond thelips of the aperture, and are folded back over the outersurface of the shell. A more developed condition is seenin species like Aplysia, in which the mantle is permanentlyreflected.1 In Limax the reflected edges of the mantle Fig. 149rt. 164 ZOOLOGY have permanently grown together. The reflection of tlit-man tie seems to be of advantage by affording additional protection to the visceral mass.But after the complete over-growth of the mantle the shellseems to be useless, and con-sequently degenerates. The Pulmonata are eitherterrestrial, like the slug, oraquatic. Of the terrestrial

Text Appearing After Image: Fis. ir,o.-shell of Helix, alboia- pulmoiiates other than the 6m, a common forest snail. Nat. slua- the most important aresize. Photo, by W. H. C. P. the snails 01 the genus Helix.1 Helix2 is noteworthy, because it is richer in species thanany other molluscan genus, since it contains over threethousand species. The distribution of the genus is world-wide. In North America the snails are most abundant inlimestone regions, consequently they are more numerousin individuals in the South and West than in graniticNew England. One of the most interesting species isHelix nemoralis, a European form, which has been intro-duced into our country at Burlington, New Jersey, andLexington, Virginia. At these places it has multipliedso rapidly, and varied to such a degree, that three hundredand eighty-five varieties have been enumerated from anarea, at Lexington, not over one thousand feet in extentin its greatest diameter3 (Fig. 151). Any species ofHelix collected in large numbers is apt to show abno

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