472 



ZOOLOGY. 



is covered underneath by the fleshy mass of the foot. The 

 viscera, lodged on the back, occupy the upper part of the 

 buckler or cone formed bv the shell, and remain always en- 

 closed in it ; but the head and foot project externally when 

 the animal unfolds itself for progression, and re-enter into 

 the last turn of the spire when they contract themselves ; 



Fig. 45i. Anatomy of a Pectinibranchiate Gasteropodons Mollusc.* 



thus, the size of this last part of the shell and the form of the 

 opening are in relation with the size of the foot. In most 

 aquatic gasteropodous mollusca in which the shell is spiral, 

 there exists a horny or calcareous disc, named operculum 

 (Fig. 454, o), which is fixed to the posterior part of tho foot, 



* Anatomy of the Turbo Pica, to show the arrangement of the respiratory 

 cavity : p, the foot of the animal ; o, operculum ; t, the proboscis ; ta, the 

 tentacles ; y, the eyes ; m, the mantle, cleft longitudinally, so as to open the 

 respiratory cavity ; f, anterior edge of the mantle, which, in the natural 

 position, covers the back, and leaves an opening or large fissure by which the 

 water reaches the gills ; b, the gills ; vb, the branchial vein proceeding to the 

 heart (c) ; ab, the branchial artery ; a, the anus ; i, the intestine ; c, sto- 

 mach and liver ; or, the oviduct. Above the nucha is the cephalic nervous 

 ganglion and the salivary glands : d, fringed membrane bordering under- 

 neath the left side of the opening of the respiratory cavity. 



