226 



THE SEA SHORE 



Spiral shells are nearly .always dextral or right-handed; that 

 is, if we trace the direction of the spiral from the apex to the 

 mouth, we find that its turns or whorls run in the same direction 

 as the hands of a watch. A few, however, are sinistral, or left- 

 handed, and occasionally we meet with left-handed varieties of 

 those species that are normally of the right-handed type. The cavity 

 of the shell is a single spiral chamber which winds round a central 

 pillar, and each whorl of the shell generally overlaps the preceding 

 one, the two being separated externally by a spiral depression 



called the suture. 



Sometimes the coils of a shell 

 are not close together internally, 

 so that the central column of the 

 spiral is hollow, and opens to the 

 exterior at the base of the shell. 

 In this case the shell is said to 

 be umbilicated, and the opening 

 referred to is the umbilicus. In 

 others the spiral winds round 

 a solid central pillar which is 

 spoken of as the columella. 



The apex of the shell, some- 

 times called the nucleus, is the 

 oldest part, and represents what 

 was once the whole. It is 

 generally directed backwards as 

 the animal crawls, and in adult 

 shells is often more or less worn 

 away by constant friction. We 

 speak of the whorls as first, second, third, &c., taking them in 

 the order of their growth, and it will generally be found that the 

 last whorl is much larger than the others, so much so that it con- 

 tains the greater part of the body of the animal ; hence this one is 

 commonly spoken of as the body-whorl, and the others make up 

 the spire of the shell. 



The mouth of the shell is of different forms in different species, 

 but in the herbivorous kinds it is usually simple, while in the 

 carnivorous species it is notched or produced. The edge of the 

 mouth (peristome) is formed by an outer lip which is usually sharp 

 in young shells and either thickened, reflected (turned outward), or 

 inflected (turned inward) in adults ; also it may be considerably 



FIG. 156. SECTION OF THE SHELL 

 OF THE WHELK, SHOWING THE 

 COLUMELLA 



