File:Image from page 182 of "Introduction to zoology; a guide to the study of animals, for the use of secondary schools;" (1900) (14781867281).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: FIG. 149. — Illustrating the transi-tion of form in the shell of certainOpisthobranchs, from the pointedspiral to the almost tiat plate. The G, Aplysia; //, Pleurobranchus.Drawn to various scales. FromCooke, Mollusca. THE SLUG AND IM ALLIED 168 .c.d ep- and greenhouses. Particularly in Europe, Limax agrestishas often devastated fields of young shoots ; this speciesis especially fond of bulbous plants. In the apparent absence of a shell the slug seems to bean aberrant gastropod. Other land gastropods — thesnails — have an evidentshell. In Limax the shellis reduced to a thin, horny ^ Jplate, embedded in themantle. Between the con-dition seen in the snailand that in Limax thereare intermediate condi-tions, in which the largeshell is partly covered bythe mantle, and othersin which the shell hasbecome reduced in size.In allies of Limax -

Text Appearing After Image: in a genus called Arioti — FIG. 149a. — Illustrating the gradualcovering of the shell (*7t) in certainOpisthobranchsby the epipodia (ep] andmantle; c. d, cephalic disc. Drawn 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 com

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