50 



MOLLUSC A. 



Inside of this skin will be seen the mantle, which 

 lined the shell, filled with various internal organs (Fig. 

 25, of). Inflate the air sac from the orifice, and ob- 

 serve the large blood vessel (y) leading to the heart, 

 and its network of branches which convey the blood 

 from the whole surface, and expose it to the air within 

 this lung-like cavity. 



Cut open the air sac by introducing the point of the 

 scissors at the orifice. The network of blood vessels can 

 be more closely examined by holding the mantle up against 

 the light, and by examining a piece under a magnifier. 



It will then be seen (Fig. 25) that each of the larger 

 branches leading to the heart has a vessel {k") from the 

 border of the mantle between it. This terminates in a 

 point, but communicates by numerous transverse tubes, 

 which can be seen by the naked eye, with the vessels (7) 

 carrying the blood to the heart. These carry a part of 

 the blood,* aerated by its passage through them, from 

 the mantle border to the heart, from whence it is distrib- 

 uted over the body. The beating of the heart, and the 

 course of the blood vessels, can often be seen from the ex- 

 terior in live snails when the shells are thin. It is con- 

 cealed on the left side in Fig. 25, but is seen in position 

 in Fig. 27, vr, an. The arterial vessels (Fig. 27, art) which 

 convey the blood from the heart are behind, and can only 

 be followed by injection and dissection. They empty into 

 the great cavity of the foot, and into the spongy mass 

 forming the face or disc below, and also into the other 

 cavities of the body, which are more or less filled by blood. 

 From these it is forced onward into large veins {k') , which 

 collect it finally into the mantle border or pulmonary vessel 

 above described, and from thence it passes through the 

 gills to the auricle of the heart (Fig. 27). 



The arrangement of the pulmonary veins (Fig. 25) are 



* A portion of the blood is carried directly to the auricle 

 without passing through the lungs or gills. 



