2 SHELL GALLERY. 



or it may be quite internal, as in the horny pen of the Squid. It 

 may be elongated, as in the Elephant Tooth-shell (Dentalium), 

 cup-shaped, as in the Limpet, or spirally twisted on itself, as in 

 the Snail. 

 l)escrip- The mantle may form a free fold on either side of the body, or 



animal. it may become largely attached to the body-wall, as in the Snail or 

 the Slag, and so give rise to an air-chamber, which, when its walls 

 are richly supplied with blood, serves as a lung. The ventral sur- 

 face of Mollusks is produced into the so-called " foot/' which may 

 be very variously modified. The foot may be more or less hatchet- 

 shaped, or curved and capable of serving as a leaping-organ, or 

 sole-shaped and adapted for creeping ; its margins may be pro- 

 duced into elongated processes, as the so-called arms of the 

 Octopus, eight in number and provided with suckers, or of the 

 Nautilus, where the arms are much more numerous, but shorter 

 and without suckers. In the Cephalopods, also, another part of 

 the foot may fold over from either side and form a median funnel, 

 through which the water of respiration is driven outwards, causing 

 the animal to move in the opposite direction — this part of the foot 

 having, therefore, still the function of an organ of locomotion. By 

 means of their muscular foot the Solenidce, or Razor-shells, burrow 

 in the sand, the Pond-Snails (Limnmdte) crawl on aquatic plants 

 and swim reversed on the surface of the water, the Limpet clings 

 to the rock, and the Cockles and Trigonias take surprising leaps. 

 The oper- Upon the upper surface of the foot, in many Gastropods, a fiat 

 hard structure termed the operculum is situated, which, when the 

 animal is retracted, partly or entirely closes the aperture of the 

 shell. In some cases, as in the Turbos, the operculum is very 

 strong and of a stony nature, but in most instances it is horny. 

 It is differently constructed in distinct families : it may be annular 

 and multispiral, annular and paucispiral, subannular and ovate, 

 or subannular and unguiculate. In Nerites it is shelly, somewhat 

 semicircular, closes the aperture of the shell, and is furnished with 

 a stout projection on the straight edge, fitting like a hinge under 

 the inner lip of the shell. A series of opercula is exhibited in 

 The Table-case G. 



breath- Thread-like processes on either side of the body, the so-called 



<rans. gill-filaments, often unite with those in front of and behind them, 



