[Illustration: FIG. 167.--A specimen of _Helix rosacea_ Müll., Cape of Good Hope, into which a piece of grass has by some means become introduced. The animal has protected itself by covering the grass with a shelly layer. (From a specimen in the British Museum.)] =Absorption of Internal Portions.=--Certain genera have the remarkable property of absorbing, when they become adult, the internal portions of the whorls and the greater part of the columellar axis. The effect of this is to make the shell, when the process is complete, no longer a spiral but a more or less produced cone, and it is found that in such cases the viscera of the spire lose their spiral form, and take the shape of the cavity in which they lie. Amongst the genera in which this singular process takes place are _Nerita_,[345] _Olivella_, and _Cypraea_ amongst marine forms, and nearly the whole of the Auriculidae[346] (Fig. 168). _Conus_ reduces the internal subdivisions of the spire to extreme thinness. It is noticeable that these genera are all of considerable thickness of shell, and it is perhaps the result of the whole energy of the animal being directed to the formation of its external protection that the internal walls of the spire become atrophied and eventually disappear. [Illustration: FIG. 168.--_Auricula Judae_ Lam., showing the disappearance of the partitions of the whorls, which are represented by dotted lines. (After Fischer.)] [Illustration: FIG. 169.--=A=, Decollated (adult) form, and =B=, perfect (young) form of _Cylindrella nobilior_ Ad., Jamaica; the dotted line shows where decollation takes place.] [Illustration: FIG. 170.--Development of _Coecum_: =A=, showing the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_, first septum; _s´s´_, second septum. (After de Folin.) =B=, Adult form of _C. eburneum_ Ad., Panama. × 8.] =Decollation.=--In certain genera, when the shell becomes adult, the animal ceases to occupy the upper whorls, which accordingly die and drop off, the orifice at the top having meanwhile been closed by a shelly deposit. Such shells are termed _decollated_. In some land genera decollation is the rule, _e.g._ in _Cylindrella_ (Fig. 169), _Eucalodium_, and _Rumina_, as well as in many species of the brackish water genera, _Truncatella_, _Cerithidea_, and _Quoyia_. _Stenogyra_ (_Rumina_) _decollata_, a common shell in the south of Europe, has been noticed to bang its upper whorls violently against some hard substance, as if to get rid of them. [Illustration: FIG. 171.--Four stages in the growth of _Fissurella_, showing how the spire gradually disappears and the marginal slit becomes an apical hole, =A=, =B=, =C=, highly magnified, =D=, natural size. (After Boutan.)] [Illustration: FIG. 172.--Three stages in the growth of _Cypraea exanthema_ L. (From specimens taken at Panama.)]