326 MOLLUSCA 



The mantle extends from the back forward over the body to near the 

 beginning of the head. It covers the spacious mantle cavity, which 

 in the water-breathing Prosobranchiata, etc., contains the gills (ctenidia) 

 and opens outward by a large aperture under the margin of the mantle. 

 The edge of the mantle may be produced into a long groove-like ciliated 

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

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

 snails branchiate respiration is replaced by pulmonate (Pulmonata, 

 Cyclostoma), which 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 blood-vessels in its dorsal wall, 

 and with only a small opening, the spiracle, on the right side. This 

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

 had degenerated, but development shows it to be an evagination arising 

 in the mantle groove. 



The visceral sac, by the great development of the gonads and liver, 

 becomes very large. Since growth downwards is prevented by the 

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

 the dorsal wall at the origin of the mantle folds, the line of least resistance. 

 Some organs, like hind gut, nephridia and heart, may be pressed into the 

 roof of the mantle cavity. 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 animal the more the spiral 

 coils and the larger the last or body whorl. 



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

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

 latter. With slight development of the visceral sac it forms a flattened 

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

 When the visceral sac is greatly elongate the shell is correspondingly 

 an elongate cone. It is rarely irregularly coiled (Vermetidae, D}. It is 

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

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

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

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

 ing umbilicus and apex, but usually they fuse into a calcareous pillar, 

 the columella, around which the whorls pass (E, r). 



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

 determines the aperture, the shell is marked with parallel lines of growth. 

 The pigment is produced 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 



