MOLLUSCS, OB SHELL-FISH. 227 



naturally expect, is close to the head at the right side 

 that is, the limpet is unsymmetrical, the organs being, as 

 it were, twisted round to the right side. 



The division of the Molluscs to which the limpets belong 

 is known as the Gasteropods. Gasteropods are usually 

 characterised by the presence of a shell, sometimes conical, 

 usually coiled, but at times absent. The mantle-fold is 

 single, and overarches a cavity which usually contains a 

 single gill ; but, as in Patella, this gill may be absent, when 

 its function is taken on by the mantle-fold. The foot forms 

 a flat surface used for creeping ; the head is distinct, bears 

 tentacles and eyes, and within the mouth there is almost 

 invariably a well -developed radula. The body is usually 

 markedly unsymmetrical, but where the shell is absent it may 

 exhibit an apparent symmetry. There are an extraordinary 

 number of Gasteropods, living on land, in fresh water, and 

 in the sea, the most familiar forms being those with coiled 

 shells, such as whelks, periwinkles, snails, and so on. 



Contrasted in many respects with the Gasteropods are 

 the Bivalves, or Lamellibranchs, such as oysters, clams, 

 mussels, cockles, etc. One may think of them in relation 

 to Gasteropods in this way. Suppose in a limpet the body 

 were to be greatly compressed laterally, the simple conical 

 shell, one might suppose, would yield to the pressure so as 

 to divide into two valves united by a hinge; the foot would 

 lose its creeping surface and become narrow and compressed, 

 the mantle-flap would grow downwards at each side, and, in 

 conformity with the two-valved shell, would become double 

 instead of single. If we suppose that at the same time 

 the separate head, the tentacles, the eyes, and the radula 

 were to be lost, symmetry to be acquired, and a second gill 

 to appear, we should have roughly indicated the chief points 

 of difference between Gastercpod and Lamellibranch. The 

 latter are much more sedentary than the former, which 

 usually live buried in sand or mud, and show fewer varia- 

 tions in structure. All feed on microscopic food particles 

 in the water, and have large flat plate-like gills, whence 

 the name of Lamellibranch, and have a double shell, whence 

 the alternative name of Bivalve. 



The third great set of Mollusca includes the active pre- 

 daceous cuttles, which are known as Cephalopoda. In 



