294 MolUixca. 



Gastropods, the shell-tube is coiled in siu^li n. wny that a 

 large cavity is left in the middle, surrounded by the turns of 

 the shell, and open below ; usually, however, the coila lie upon 

 one another, so that this cavity, the u m bili cal-tubo, is very 

 narrow; and frequently, its outer opening, the umbilicus, is 

 closed by the last coil. Those portions of the spiral which im- 

 mediately surround the umbilical-tube, form a kind of central column, 

 the columella. 



Growth of the .shell occurs as follows : new material is secreted 

 by the thickened edge of the mantle, and is deposited at the rim of 

 the aperture, and thus increase in length takes pLice; growth appears 

 to be intermittent, in the course of a short time a large poi'tion is 

 added, then a longer period of rest occurs, and so on. The portion 

 of newly-formed .shell substance, which covers and growH on to the 

 older coils, is usually thinner than the remaining free portion, and is 

 often difficult to make out. In addition to this do[)o.sit at the mouth, 

 calcareous material is also laid down within the shell, over thu 

 whole surface of the visceral mass, and in this way an increase in 

 thickness is effected. The small, oldest coils at the apex of the 

 shell may, by this nutans, become filled with lime : occasionally such 

 portions get broken away. J^osides the new formation of calcareous 

 material, -a r e a b s o r p t i o n of the older part frequently occurs ; 

 for instance, it may occasionally be observed that before the period of 

 growth begins, superficial portions of the old shell, near to the 

 mouth, become loosened and worn away where the new shell will 

 later be deposited ; internally, too, an absorption of the concealed 

 portions of the shell tiikc^s place, by means of which the septa between 

 successive coils may be much thinned, or, completely dissolved. The 

 Gastropod shell consists chiefly of calcium carbonate, with a small 

 amount of conchiolin; there is usually a thin, uncalcified horny layer, 

 superficially, which is easily removed. 



Transverse sections of shells vaiy in form, they are rarely cirouliu', usually 

 somewhat compressed, occasionally even elliptical. In Home instances, the younger 

 coils may almost, or entirely, sniTound the older ones, so that the latter are 

 nearly, or completely, hidden from without. The axis of the conical shell is 

 sometimes long, sometimes short ; in the latter case, the shell approaches a discoid 

 form. In some cases the coils are very numerous, and the croHH-Hcc.tifm of the 

 tube increases gradually in size; in others there iirc only a few turns, which 

 increase rapidly in diameter. In not a few Gastropods, when growth has ceased, 

 the mouth of the shell takes on a peculiar form, becomes thickened, and widened j 

 in others, even in young animals, there is a peculiar i-im (thickened and 

 rough), round the mouth and at the close of each period of gi'owth a new rim is 

 formed, so that the old mouths are indicated by special regions in the shell. (For 

 the notch in the shell foi- the bianchial syphons, tee below). The shells are often 

 brightly coloured, and rough with iinor or coarser sculpturings on the surface, 

 etc. Sometimes the edge of the mantle is specially well develojicd, wraps over the 

 edge of the shell, and secretes a bright layer over the surface {e.<j., in Cowries, 

 Cypnea). 



