GASTEROPODA. 397 



infinite number of minute lobules, every one of winch produces 

 a biliary vessel ; and these, joining continually with each other, 

 form four large hepatic ducts, one proper to every lobe of the liver. 

 The four hepatic ducts ultimately unite into one great central 

 vessel (c), that opens into the alimentary canal in the immediate 

 vicinity of the pyloric extremity of the stomach. 



(433.) The genus of Gasteropoda to which the Snail belongs 

 is composed of air-breathing animals, and we must accordingly ex- 

 pect to find these mollusca provided with a respiratory system 

 specially adapted to the mode of life to which they are destined. 

 The mechanism adopted is as follows : A capacious chamber, of a 

 somewhat triangular form, is found placed beneath the dorsal sur- 

 face of the body, and separated from the visceral cavity by a broad 

 muscular septum forming its floor. Into this chamber a wide 

 orifice (j%. 195, e ), placed upon the right side of the body near 

 the margin of the shell, allows the atmospheric air to enter. 

 The roof of the respiratory cavity is covered with a most intricate 

 arborescence of blood-vessels rudely sketched in Jig. 183, Ar, in 

 which the blood is freely exposed to the air therein contained ; 

 while the muscular floor, performing alternate movements analogous 

 to those of the human diaphragm, continually draws in and expels 

 the air, so as to ensure its constant renewal. The manner in 

 which respiration is effected, and the general disposition of the cir- 

 culatory apparatus, is therefore briefly this : The blood derived 

 from all parts of the body is brought to the respiratory chamber by 

 large veins provided for the purpose ; arrived there, it is dis- 

 persed through the countless ramifications of delicate vessels spread 

 over the entire roof of the breathing cavity, and thus becomes ex- 

 posed to the purifying influence of oxygen. The renovated blood 

 is then re-collected by the large pulmonary vein (k) ; and being 

 conveyed to the heart, which is composed of a single auricle (h) that 

 communicates with a strong ventricular cavity (g), it is propelled 

 through the entire arterial system derived from the aorta (f). 



(434.) The whole of that part of the body of the snail which is 

 not permanently covered by the shell is defended by a thick skin, 

 the surface of which is irregularly furrowed, and continually moist- 

 ened by a viscid secretion that exudes from glands apparently 

 imbedded in the substance of the integument ; and the tenacious 

 slime so furnished, if the creature be irritated, is poured forth in 

 astonishing abundance. 



Nevertheless, besides the slimy material thus copiously supplied 



