CIRCULATION AND RESPIRATION OF GASTEROPODS. 359 



animal, and is lodged in the last turn of the spiral shell, when 

 the Mollusk is provided wi th an envelope of this kind. Among 



Fig. C52 ANATOMY OF TURBO PICA : p, foot ; o, operculum ; t, proboscis ; ta, tentacula , 

 y, eyes ; m, mantle opened longitudinally, to show the disposition of the respiratory cavity ; 

 /, anterior border of the mantle, which, in its natural position, covers the back of the ani- 

 mal, leaving a wide slit by which the water enters the branchial cavity; b, the gills; c&, 

 branchial vein, returning to the heart, c; ab, branchial artery; a, anus; i, intestine; e t 

 stomach and liver; or, oviduct. On the upper side of the neck are seen the cephalic gang- 

 lion, and the salivary glands. And at d is shown a fringed membrane, which forms the 

 lower border of the left side of the opening that leads to the respiratory cavities. 



those Gasteropods destined to breathe in water, the arrangement 

 of the gills varies considerably ; in many, these organs are lodged 

 in a cavity analogous to that 

 which constitutes the lung of a 

 the preceding (Fig. 652) ; but 

 in others, they are placed be- 

 tween the mantle and the foot, 

 or even on the back of the ani- 

 mal, so as to float freely in the '' 

 surrounding liquid. As ex- 

 amples of the pulmonic Gas- 

 teropods, we may mention the 

 Snail, and the Slug, which live on land ; and the Lymnasn, the 



FIG. f.53. PLETJROBRANCHU8 : m, the mantle 

 turned back to show the gills, ! : a, the anus ; 

 b, mouth and proboscis ; t>, hood ; t, tentacula ; 

 p, foot. 



