Introduction to Animal Morphology. 285 



and is called the nucleus. The line of contact of suc- 

 cessive whorls is the suture. The aperture at the end 

 of the last, or body whorl, is surrounded by a variably 

 shaped margin [pcritrcinc], which may be circular, 

 oval, semicircular, angular, spiny, or ridged, according 

 to the shape of the mantle edge. The pillar along 

 which the whorls are coiled is called the columella,* 

 and it may be perforated below by a canal (Umbilicus), 

 or covered by a thick shelly deposit (Umbilical 

 callus).! The part of the last w^horl overlapping the 

 columella is the columellar lobe. The apex is turned 

 backwards (except in Patella), sometimes breaks off 

 in the adult, producing decollate shells (Subulina 

 [Rumina] decollata, Urocoptis). The peritreme is 

 entire in vegetable feeders, or its anterior end, or both 

 ends, may be prolonged into siphons, which are 

 formed by long, gutter-shaped processes of the mantle. 

 In Typhis and Haliotis the shell is pierced by holes, 

 corresponding to deficiencies in the mantle edge. 

 The shell in some cases is rudimental, contained 

 within the mantle (Limax, Parmophorus), and then it 

 may be soft, or else glassy, like that of Cymbulia 

 (Marsenia), or made of calc-spar-like material (x\rion). 

 The operculum may be membranous, horny, or 

 shelly, and usually is of the form of the generating 

 plane of the spiral, and keeps itself geometrically con- 

 stant by rotating on its own axis while revolving 



* Rarely the whorls may be disjointed, as in Vei-metus where there is 

 no columella ; or the entire wall of the chamber may be perfect without 

 confluence with its contiguous neighbour, as in Scalaria where there is 

 also no columella. In convolute shells lilve Cji^rsea the peritreme may run 

 along the whole length. 



t The shape of the shell may vary in the two sexes ; the female shell 

 having the last, or body whorl, A\adcr than the male. 



