_The Slit._--Many shells are furnished with a slit in the last whorl, which opens, in most cases, on the outer lip, and is sometimes of considerable depth, at others a mere notch. In the patelliform shells it is always in front of the apex. The function of the slit appears to be mainly anal, the excretory products being thus allowed to escape by a passage of their own, without soiling the clean water taken in by the branchiae. The posterior canal of some Gasteropoda probably performs a similar function. In the adult _Fissurella_ the slit becomes an apical hole (see Fig. 178 F), in the allied genera it is either immediately in front of the spire (_Puncturella_), or half-way between the spire and the anterior margin (_Rimula_), or on the margin and well marked (_Emarginula_), or a mere indentation of the margin (_Hemitoma_). In _Pleurotomaria_ it is exceptionally long, and is well marked in _Bellerophon_, _Schismope_, _Scissurella_, _Murchisonia_, and _Pleurotoma_ (where it is sutural). In _Haliotis_ and _Polytremaria_ it is replaced by a series of holes, which are closed up as the animal grows past them. Some of these holes (at least in _Haliotis_) certainly serve the purpose of admitting water to the branchiae, while others are anal. In _Trochotoma_ there are only two holes, united by a narrow fissure. _The Tubed Land Operculates._--A group of the Cyclophoridae, which is restricted to Further India and the great Malay Islands, has developed a remarkable _sutural tube_ on the exterior of the last whorl, near the aperture, A similar tube, but more obscure, exists in _Alycaeus_. Several stages in the development of this tube may be noticed, beginning with the elevation of part of the peristome into a simple irregular shelly plate, which is continued, first into a short, and then into a long tube, which becomes soldered to the shell; finally, the tube becomes free, and the anterior part of the last whorl is disconnected from the spire (Fig. 180 A-D). [Illustration: FIG. 179.--The slit in =A=, _Bellerophon_, =B=, _Pleurotomaria_, =C=, _Schismope_, =D=, _Polytremaria_, =E=, _Haliotis_ (not drawn to scale).] [Illustration: FIG. 180.--Development of the tube in the tube operculates: =A=, _Pterocycus rupestris_ Bens.; =B=, _Opisthoporus birostris_ Pfr.; =C=, _Spiraculum travancoricum_ Bedd.; =D=, _Rhiostoma Housei_ Pfr.] It is singular that the tube does not appear to be of any use to the animal, since its internal extremity, in the complete form, is closed, and does not communicate with the interior of the whorl. It may be presumed, however, that in origin the tube served as a means of conveying air to the animal when the operculum was closed. The holes in the peristome of _Pupina_, _Cataulus_, and _Anostoma_ (Fig. 154) may be compared. [Illustration: FIG. 181.--_Eburna spirata_ Lam., E. Indies. =F=, foot; =OP=, operculum; =P=, penis; =S=, siphon; =T=, tentacles, with eyes at their base. (After Souleyet.)]