RESPIRATORY CHAMBER. 525 



up of four lobes (fig. 264, b, d) of a dark-brown colour, and composed 

 of an infinite number of minute lobules, every one of which 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. 



(1386.) The genus of Gasteropoda to which the Snail belongs is 

 composed of air-breathing animals ; and we must accordingly expect 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 surface of the body, and separated 

 from the visceral cavity by a broad muscular septum forming its floor. 

 Into this chamber a wide orifice (fig. 261, 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 intri- 

 cate arborescence of blood-vessels (rudely sketched in fig. 264, &), 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 circulatory 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 dispersed through the countless ramifica- 

 tions of delicate vessels spread over the entire roof of the breathing 

 cavity, and thus becomes exposed to the purifying influence of oxygen. 

 The renovated blood is then re-collected by the large pulmonary vein 

 (Ar) ; and being conveyed to the heart, which is composed of a single 

 auricle (h) that communicates with a strong ventricular cavity (</), it is 

 propelled through the entire arterial system derived from the aorta (/). 



(1387.) 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 moistened 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. 



(1388.) Nevertheless, besides the slimy material thus copiously sup- 

 plied by the tegumentary glands, there is in the interior of the animal 

 a special apparatus apparently destined to furnish a viscid fluid of a 

 similar character. The gland alluded to, called by Cuvier*, par excel- 

 lence, " the secerning organ of the viscosity," is in the Snail a triangular 

 viscus (fig. 264, i) placed in immediate contiguity with the pericardium. 

 * Histoire des Mollusques Memoire sur la Limace et le Colima^on. 



