GASTROPODA. 361 



Lanistcs, and a few other forms the shell is sinistral, but the organization is 

 dextral ; such shells are said to be ultra-dextral (Pelseneer). In some cases 

 both sinistral and dextral species are known in the same genus. 



As a general rule the whorls of the spire are closely applied together, the line 

 or groove of apposition being called the suture. Sometimes, however, the whorls 

 are more or less separate (Scalaria, Cyclostoma, Valvata, Vermetus). The shell 

 is never multilocular, , but sometimes the animal ceases to occupy, the upper 

 whorls, which drop oft' (decollation). The axis round which the whorls are coiled 

 is called the columella ; it may be hollow, in which case the shell is said to be 

 perforated or umbilicated (Solarium) and the axial tube is called the umbilicus; 

 the umbilicus may be shallow or deep. In Natica the umbilicus is filled up 

 with calcareous matter. In other shells the whorls are closely coiled, and 

 the columella is a central pillar ; such shells are imperforate. In addition to 

 the above the following terms are applied to the shell : the apex is the top 

 of the spire the first formed part of the shell ; the aperture is the opening 

 of the shell at the end of the last, or body-whorl. The aperture is entire, 

 as in most vegetable feeders (holostomatous), or produced at its anterior 

 end into a spout (canal), as in many carnivorous families (siphonostomatous). 

 Sometimes there is a posterior canal as well ; this is anal in function, and in 

 some forms is represented by the so-called slit (Emarginula, Pleurotomaria, 

 Scissurella). The slit lies over a slit in the mantle (see above). In Fissurella 

 (Fig. 281) it becomes in the adult an apical pore, and in Haliotis a number 

 of pores. 



The margin of the aperture is called the peristome, the right side of which is 

 the outer lip or labrum, the inner or columellar lip being the labium. The 

 outer lip is sometimes thin, more often thickened, or reflected or inflected 

 (Cypraea), or expanded (Pteroceras) or fringed with spines (Murex). When the 

 fringes or expansions of the outer lip are formed periodically during the growth 

 of the shell they are called varices. 



Shells which are always covered by the mantle are colourless (Limax, 

 Parmophorus). Those which are covered by the mantle when the animal 

 expands acquire a glazed or enamelled surface like the cowries. When the 

 shell is deeply immersed in the foot of the animal it becomes partly glazed 

 (Cymba). In all other cases there is an epidermis or periostracum, though it is 

 often thin and transparent. 



In some cases the parts of the shell separating the successive whorls and the 

 columella are absorbed in the adult, e.g., Nerita, Olivella, Cypraea, Auriculidae; 

 in such cases the visceral sac loses its spiral form. 



The operculum is attached to the dorsal hind end of the foot. It consists of 

 a horny basis which may be calcified. It grows with the shell, and the oldest 

 part of it is called the nucleus. It is marked by lines of growth, which may be 

 concentric or spiral (siuistral in dextral shells). Sometimes it is claw-shaped 

 or unguiculate, in which case the nucleus is apical. It generally fits the mouth 

 of the shell ; but is sometimes too small, or even ridiculously inadequate for this 

 purpose, as in Bullia, Conus, etc.; it also persists in some limpet-like forms 

 which adhere by their flat foot, and in which it is not used (Navicella, Conclio- 

 lepas, Sigaretus). In such cases the operculum affords a good instance of an 

 organ persisting after it has entirely lost its original function. The operculum 

 is present in most adult Streptoneura, and in their larvae, if absent in the adult 

 (except Stilifer) ; it is absent in the adults of most Euthyneura (except Actaeon, 

 Limacina, Amphibola), but is present in their development except in some of 



