3io COMPARATIVE ANATOMY AND PHYSIOLOGY. 



and sometimes impregnated with calcareous salts ; it 

 is very generally distributed among the Gastropoda, 



. 127. A, Section of Spirula australis, to show the internal shell ; B, 

 Belemnite restored ; c, Pen of the Loligo ; u, Argonauta in Natural 

 Position. (From Woodward.) 



and may completely fit the mouth of the shell, as in 

 the common snail, or close part only of it, as in 

 Strombus; or, as in Doliurn and other large-mouthed 



