718 



ZOOLOGY 



SECT. 



v ,iF- r ? m x 



Natural History.) 



such spiral shells are dextral, i.e. if we begin at the apex of the 



spiral to reach the opening of the shell 

 we have to pass from left to right, with 

 the columella always on our right-hand 

 side : in a few cases, however, the spiral 

 is sinistral, taking the opposite direction 

 from that of the ordinary dextral shell. 

 The form of the shell varies with 

 the degree of obliquity with which the 

 whorls are set on the axis. When 

 the obliquity is very slight (Fig. 630) the 

 spiral is nearly flat ; when the obli- 

 quity is great, an elongated tapering 

 sh ell such as that represented in Fig. 631 

 the Cambrid 'J e is the result. Sometimes the later whorls 



. . ., 



completely cover over the earlier ones, 

 so that the spiral form of the shell is concealed. Sometimes only 

 the apical portion of the shell is spiral, the re- 

 mainder being a straight or sinuous cylinder. 

 The spiral form of the shell and the parts enclosed 

 in it, as well as the direction of the spiral, 

 whether dextral or sinistral, are, it may be here 

 pointed out, very fundamental features of the 

 organisation of the Gastropod, and are fore- 

 shadowed at an early stage in the segmentation 

 of the ovum. The mouth of the shell has usually 

 a prominent margin or peristome, which is some- 

 times entire and continuous, sometimes broken 

 by a deep notch or a spout-like process or canal, 

 formed in connection with the development of a 

 spout-like prolongation of the mantle, the siphon, 

 which lies in it. The mouth of the shell in many 

 Gastropoda is capable of being closed by means of 

 an operculum 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. The margin of the mantle 

 in some cases bears a series of tentacles. Lateral 

 folds of the mantle are in some of the Gastropoda 

 (Fig. 632) reflected over the shell and may com- 

 pletely 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 FIQ 631 _ shell 

 Apltjsia and some other Opisthobranchs the shell Terebra ocuiata. 





