370 MOLLUSC A. 



The mantle begins on the back and extends thence forward 

 over the body to near the beginning of the head. It covers the 

 mantle cavity, a spacious chamber, 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 

 siphon, conveying water to and from the branchial chamber, 

 which is of importance in determining the shape of the shell. 

 When, by degeneration of the gill, the animals become air-breath- 

 ing, the mantle cavity becomes a lung, and the opening, by growth 

 of the mantle edges to the body, becomes a small spiraculum, 

 closed by muscles. 



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 nephridia and heart, 

 may be pressed into the mantle cavity. When the visceral sac, as 

 often occurs, becomes enormous, it does not stand directly upwards, 

 but coils from left to right in a spiral. The older the animal 

 the more the spiral coils and the larger the last or body whorl. 

 The visceral spiral therefore begins at the tip with narrow whorls 

 which increase in size with approach to the rest of the body. 



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

 As a secretion of the mantle it takes the form which the mantle 

 assumes under the influence of the visceral sac. With slight devel- 

 opment of the visceral sac it forms a flattened cone (fig. 362, 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, fig. 

 362, 0). 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 (fig. 362, D, E) and one can speak of its apex and base. 

 In the middle of the base is usually a depression, the umbilicus. 

 Sometimes the coils are loose and do not touch in the axis con- 

 necting umbilicus and apex, so that one can look into the space, 

 but usually the coils fuse together into a calcareous pillar, the 

 columella, around which the whorls pass (fig. 362, E, c). 



The shell increases to a certain size by additions from the mantle 

 edge; and since this determines the aperture, the shell is marked 

 with parallel lines of growth. The pigment is elaborated on the 

 edge of the mantle, and in the formation of the shell passes into 



