516 



ZOOLOGY. 



initiated anteriorly by a head more or less developed, in which 

 is the mouth, provided with fleshy tentacles, varying in num- 

 ber from two to six ; the back is covered with a mantle pro- 

 longed more or less backwards, under the form of a mem- 

 branous sac, and which secretes the shell ; finally, the belly 

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



vb ab ob ov 



Fig. 540. Anatomy of a Pectinibranchiate Gasteropodous Mollusc.* 



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

 buckler or cone formed by 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 ; 



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

 cavity : p, the foot of the animal ; o, operculum ; t, the proboscis ; in, 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 ; ov, 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. 



