22 THE LIFE OF THE MOLLUSCA 



a trap-door (Plate VIII., Fig. 2), the " operculum "; 

 when extended, this lies on the animal's back, and 

 in many cases forms a sort of pillion on which the 

 shell rests (Plate II., Fig. 1, and Plate VIII.,. Fig. 1). 

 It consists of a horny layer, sometimes strengthened 

 by the addition of shelly matter, which differs in 

 structure from that of the shell itself. Its inner 

 side is marked by the scar of the muscle which is 

 attached to it, while outside it exhibits lines of 

 growth. The operculum, various forms of which 

 are shown on Plate VIII., Figs. 4-13, grows by 

 additions made to the original point of beginning, 

 the " nucleus," either all round, concentrically: or 

 on one side, so that the nucleus remains at the end 

 or at one side ; or the growth may result in a spiral. 

 This spiral is sinistral in dextral, dextral in sinistral 

 shells. Nearly all the Gastropoda are furnished 

 with an operculum in the young stage within the 

 egg, and though some discard it on hatching out, 

 the greater number retain it throughout life. Many 

 of the spiral and concentric forms fit the mouth of 

 the shell accurately, others only partly close the 

 aperture, in yet others it becomes rudimentary, while 

 in certain specialized forms, such as S trombus and 

 its allies, it is converted into a sort of claw at the 

 end of the elongated foot, and is used to assist the 

 animal in progression (Plate VIII., Fig. 3). 



A more peculiar means for blocking the entrance of 

 the shell when the animal retires into it is possessed 



