THE SLIT 



265 



whole spire (see Fig. 177, which also shows how successive fresh 

 growths have thickened the columella). 



The whorls may be wound in a spiral, which is either hollow, 

 as in Solarium, or quite compact, as in Oliva^ Terehra, Cypraea^ 

 with every possible intermediate grade. This concavity, whicli 

 varies in depth and width, is known as the umhilicus^ and shells 

 are accordingly spoken of as deeply {e.g. most Trochidae and 

 Naticidae), narrowly (e.g. Lacuna^ Littorina')^ or widely (e.g. 

 Solarium) umhilicated. When the spiral is quite flat, as in 

 Planorhis and some Helix^ the umbilicus vanishes entirely. 

 Shells in which the whorls are so compactly coiled on an 

 ascending spiral that there is no umbilicus, are termed imper- 

 forate. 



The Slit. — Many shells are furnished with a slit in the last 

 whorl, which opens, in most cases, on the outer lip, and is some- 



FiG. 178. — The slit in A, Hemitoma, B, Emarginula, C, Macroschisma, D, Craniopsis, 

 E, Puncturella, F, Fissurella. 



times 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 Avell marked 

 {Emarginida)^ 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 



