686 



ZOOLOGY 



SECT. 



The shell in the adult limpets (Patella and allied genera) is in 

 the form of a short cone. In most of 1 he Gastropoda it is in the 

 shape of a spiral with the turns usually in close contact with 

 one another, the inner walls of the turns coalescing to form an 

 ;axial, hollow or solid column the columella. The portion of 

 the shell projecting inwards between the turns of the spiral 

 .sometimes becomes absorbed. In certain cases, on the other hand, 



the cavity of the apical portion of the spiral 

 may become cut off from the cavity of the 

 rest of the shell by the formation of a trans- 

 verse partition, the animal then becoming 

 restricted to the basal portion. By far the 

 greater number of 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. 582) the 

 spiral is nearly flat ; when the obliquity is 

 great, an elongated tapering shell such as that 

 represented in Fig. 583 is the result. Some- 

 times 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 mouth of the shell has usually a promi- 

 nent margin or peristome, which is sometimes 

 entire and continuous, sometimes is broken 

 by a deep notch or a spout-like process 

 or canal, formed in connection with the de- 

 velopment 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 tentacle*. 

 Lateral folds of the mantle are in some of the Gastropoda 

 (Fig. 584) reflected over the shell and may completely cover it, 

 In some cases these folds unite by their edge, so that the shell 



FIG. 583. Shell of 

 Terebra oculata. 



