. Text-book of zoology for junior students. Zoology. I-ig. ]4::. — Scuhir Cruuihn ilin,, a II Instnina ..1113 Ul be noticed. The shell is composed either of a single piece (univalve), or of a nnraber of ])lates jilaced tine behind the other (multivalve). The univalve shell is to be liioked upon as essentially a hollow cone, the ape.'c of which is placed a little on one side. In the simplest forms, as in the Limpets, the conical shell is retained throughout life without any alteration. In the great majority of cases, how- ever, the cone is considei-ably elongated, so as to form a tube, which ma
. Text-book of zoology for junior students. Zoology. I-ig. ]4::. — Scuhir Cruuihn ilin,, a II Instnina ..1113 Ul be noticed. The shell is composed either of a single piece (univalve), or of a nnraber of ])lates jilaced tine behind the other (multivalve). The univalve shell is to be liioked upon as essentially a hollow cone, the ape.'c of which is placed a little on one side. In the simplest forms, as in the Limpets, the conical shell is retained throughout life without any alteration. In the great majority of cases, how- ever, the cone is considei-ably elongated, so as to form a tube, which may retain this shape (as in tlie " Tooth-shell "), but whiclv is usually coiled up into a spiral. Tlie "spiral univalve" may, in fact, be regarded as the typical form of the shell in the Oaahroiioda (fig. 140, B). The coils of the sjiiial are termed the "whorls," and are usually or less on one side. In most cases, too, the wliorls are wound obliquely round a central axis or pillar, increasing gradually in size to the mouth. The last whorl is the largest, and is termed the " ; The mouth of the shell in many forms is uidjrolcenly I'ound or " entire " (ti:hi are divided into a giiod many groups, of which the more imjiortaut may be briefly' noticed, the foregoing a|iplying chiefly to the ordinary forms, which therefore need no fuither description. The remaining mendjers of the water- breathing (Jastero|iods are divided into two sections, ihH'ering a gond deal the ty]iical forms of the in many respects. As exam|iles of tlie tirst (if these may be taken the Sea-slugs and Sea- lemims {Xiulihruin'liinhi), s[iei;imeus of which may at any time be fninid ci'eepiug abnut on sea-weeds, or attached to the under sur- fare of stones at low These slug-like animals (tig. 144) are wholly ili'xtitnle of a shell when fully grown, but possess an em- bryonic shell when young. When there are any d
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1885
