2 SHELL GALLERY. 



which, when its walls are richly supplied with blood-vessels, serves 

 as a lung. The ventral surface of Molluscs 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 produced 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 loco- 

 motion. By means of their muscular foot the Solenidce, or Eazor- 

 shells, burrow in the sand, the Pond-Snails (Limnceidce.) 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 flat 



"*" "™" 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, it 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 the 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 opercida is exhibited in side 

 table-case C. 

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



breathing giU-filaments, often unite with those in front of and behind them, 

 organs. ^^^ ^^ ^^^^ ^.^^ ^^ plates ; these, when well developed, are best 

 seen in the division to which the Oyster and the Mussel belong. 

 Where the body is coiled or twisted on itself, as so often happens, 

 the gills of one side may be altogether lost. Sometimes, as in 

 Pliyllirhoe, when the body is small and its wall thin, the gills 

 (ctenidia) disappear altogether, and there is no special breathing- 

 organ ; in others the loss of the gill is compensated for by the 

 formation by the mantle of a lung ; this is most often seen in the 

 forms that live on land. 



