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 (pcntrciuS, 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 maybe perforated below by a canal (Umbilicus), 

 or covered by a thick shelly deposit (Umbilical 

 callus). t The part of the last whorl 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 

 , may be prolonged into siphons, which are 

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

 In Typhis and llaliotis 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 Arion). 

 The oporculum may be membranous, horny, or 

 sholly, and usually is of the form of the ovn 'rating 

 plan<- piral, and keeps itself geometricall] 



stant by rotating on its own axis \vli Iving 



. 



no coll.; \\ithout 



conflu< : 

 also no 



:ttll. 



t 'I :. the shell ii. > ihdl 



