362 CIRCULATION AND RESPIRATION OP GASTEROPODS. 



903. The heart is always systemic (ANIM. PHYSIOL. 281), 

 and is composed almost invariably of a ventricle and an auricle 

 (Fig. 539) ; it is placed near the back of the animal, opposite 

 that side occupied by the reproductive organs. The organs of 

 Respiration are formed in some instances for aerial, in others for 

 aquatic, respiration. In the first case they consist of a cavity, 

 on whose walls the blood-vessels form a complicated net-work ; 

 and into the interior of which the external air penetrates, through 

 an orifice in the outer border of the mantle. This pulmonic, or 



vb ab b ov 



FIG. 555. ANATOMY OF TURBO PICA : p, foot , o, operculum ; t, proboscis ; ta, tentacula ; 

 /, 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 animal, leaving a wide slit by which the water enters the branchial cavity ; b, the 

 gills ; vb, branchial vein, returning to the heart, c ; ab, branchial artery ; a, anus ; 

 t, intestine ; e, stomach and liver ; ov, oviduct. On the upper side of the neck are seen 

 the cephalic ganglion, 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. 



lung-like cavity (Fig. 564), is situated on the back of the 

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

 the Mollusk is provided with an envelope of this kind. Among 

 those Gasteropods destined to breathe in water, the arrangement 

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



