320 MOLLUSCA 



The mantle extends from the back forward o-\-er the liody to near the 

 beginning of the licad. It covers the spacious manth' ca\ity, wliich 

 in the water-breatlting rrosobrancliiata. etc., contains the gills (ctenidia) 

 and opens outward bv a large aperture under the margin of the manlh\ 

 Tire edge of the mantle ma\' be ptroduced into a long groove-like ciliated 

 siphon, conveying water to and from the branchial chamber, and is of 

 importance in determining the shape of tlie shell. In the terrestrial 

 snails branclriate rcs])iralion is replaced b)- pulmonate (Pulmonata, 

 Cy(7().\7();;;i7), wliich is retained in many forms (Basommatophora) which 

 have returned to an aquatic life. In place of the mantle cavity there is a 

 sac filled with air, with a rich network of bhuxl-xessels in its ilorsal \\all, 

 and with only a small opiening, the spiracle, on the right side. Tlris 

 lung was formerly thought to be the mantle cavity in wlrich the ctenidium 

 had degenerated, but ile\-elopment shows it to be an evaginalion arising 

 in the mantle groo\e. 



The visceral sac, bv the great development of the gonads and lixer, 

 becomes very large. Since growth do\\']twards is prevented b\' the 

 muscular foot, the organs press towards the back, carrying before them 

 the ck^rsal wall at the origin of the mantle folds, the line of least resistance. 

 Some organs, like hind gut, nephridia and heart, mav l>e pressed into the 

 roof of the mantle ca\ily. When the visceral sac, as often occurs, 

 becomes enormous, it does not stand directly upwards, but coils, usually 

 from left to right, in a spiral. The older the aitimal the more the spiral 

 coils and the larger the last or body whorl. 



From the foregoing the shape of the shell is easilv understood. As a 

 secretion of the mantle and the visceral sac it takes the form of the 

 latter. Widi slight development of the visceral sac it forms a tlallened 

 cone (fig. 330, .1), or is slightly coiled at the apex, as in the abalone (B). 

 ^^'hen the visceral sac is greatly elongate the shell is correspondinglv 

 an elongate cone. It is rarely irregularly coiled (Xermetida-, P). It is 

 usually coiled like a watch spring in one piano, or like a spiral staircase; 

 in the latter case the shell is more or less conical (C", J\) and one can 

 speak of its apex and luise. In the middle of the base is usually a ilepres- 

 sion, the in)ibilinis. Sometimes the coils do not touch in the axis connect- 

 iitg umliilicns and apex, Iml usually ihey fu.se into a calcareous pillar, 

 the roluindia, around which the whorls pass {l\, c). 



The slrell increases by additions from the mantle edge; and since this 

 determines the aperture, (he shell is marked with jia'-allel lines of growth. 

 The pigment is proiluced on the edge of the mantle, and passes into 

 the shell as formed, causing its color pattern. When the siphon is pres- 

 ent the shell shows a corresponding process. Thus are distinguished 



