. A manual of zoology. 2SS MANUAL OF ZOOLOGY sect. borne on the foot. In some terrestrial forms in which an operculum is absent, the opening may be closed up during winter by a layer of hardened mucous matter to which the name of epiphragm is applied. Lateral folds of the mantle are in some of the Gastropoda (Fig. 176) reflected over the shell, and may completely cover it. In some cases these folds unite by their edge, so that the shell comes to be enclosed in a complete sac of the mantle; such enclosed shells are always imperfectly developed and incapable of covering the body. Thus in Aplysia
. A manual of zoology. 2SS MANUAL OF ZOOLOGY sect. borne on the foot. In some terrestrial forms in which an operculum is absent, the opening may be closed up during winter by a layer of hardened mucous matter to which the name of epiphragm is applied. Lateral folds of the mantle are in some of the Gastropoda (Fig. 176) reflected over the shell, and may completely cover it. In some cases these folds unite by their edge, so that the shell comes to be enclosed in a complete sac of the mantle; such enclosed shells are always imperfectly developed and incapable of covering the body. Thus in Aplysia (the " Sea-hare ") and. Fig. 174. — Shell of Solarium perspectivum from the under side. (From the Cambridge Natural History.') other allied forms the shell is greatly reduced, thin and horny, and concealed within the mantle, while in the nudibranch (Fig. 177), members of the same sub-order, it is entirely absent. The shell is also completely absent in some of the pelagic forms [Heteropoda and Pteropodd); in others, though present and external, it is too small to enclose the animal. In the slugs the shell is vestigial and concealed by the mantle. The Gastropoda have a well-marked head, separated from the body by a constriction or neck. The mouth,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Parker, T. Jeffery (Thomas Jeffery), 1850-1897; Haswell, William A. (William Aitcheson), 1854-1925. New York, The Macmillan Company; London, Macmillan & Co. ,Ltd.
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyo, bookyear1900
