304 



IXVEIiTEBRATE ANIMALS. 



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 

 m. ire or less amalg.-imated 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 " liody-wliorl." The mouth 

 of the shell in many forms is uidjrolcenly I'ound 

 or " entire " (ti<'. 143), and it is found that mott 

 of these forms subsist on vegetalile food, as, 

 fur instance, the common Periwinkles. In others, again (fig. 140, 

 11), the miiuth nf the .shell is notched or is produced into a canal, as 

 in the commim "Whelk, and it is found that tliese live upon animal 

 food, or aie " carni\'oiiius." There may be more than one of these 

 canals or tubes, but thi-y ihi not necessarily indicate the nature of 

 tlie food, as their funi-tiim is to jirotect the resjiiratory siphons. 



The (liixti'r(iiii>: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 

 fr.ini the ty]iical forms of the cla.ss in 

 many respects. 



As exam|iles of tlie tirst (if these 

 may be taken the Sea-slugs and Sea- 

 lemims {Xiulihru in'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 wat.er. 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 distinct respiratory 



