A specimen of Helix rosacea, in the British Museum, into whose shell a piece of grass somehow became introduced, has partitioned it off by the formation of a sort of shelly tunnel extending throughout its entire length (Fig. [167]).
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.
Fig. 168.—Auricula Judae Lam., showing the disappearance of the partitions of the whorls, which are represented by dotted lines. (After Fischer.)
Fig. 169.—A, Decollated (adult) form, and B, perfect (young) form of Cylindrella nobilior Ad., Jamaica; the dotted line shows where decollation takes place.
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.