LAWSON, ON LIMAX MAXIMUS. 19 
and this cavity is termed the lung. The respiratory organ 
is usually described as being a ring surrounding the heart ; 
this, however, is not correct; it is a double sac, one pocket 
of which is situate on the right, and the other on the 
left side, having two channels of communication, by means 
of which the air is conveyed to every portion of the vascular 
surface. These pouches are placed in the thoracic region of 
the body (fig. 5), and are constituted externally of the general 
integument, and within of a delicate fibrous membrane, 
which also serves to define their limits; their upper borders 
are bounded by the inferior surface of the shell, and below 
they are separated from the viscera by a septal fold of their 
inner membrane, which also forms a posterior partition be- 
tween the lung and abdomen; anteriorly they are closed in 
by the same structure, and internally they are related to the 
heart and pericardial gland, which are placed between the 
two sacs. The connecting channels cross the body, one in 
front of the pericardial gland and heart, and the other im- 
mediately behind them. The air is admitted through an 
orifice of an elliptical or doubly cuneate form, which is upon 
the right side near the middle lateral line, and at about 
4 inch from the right upper tentacle. The great veins which 
convey the blood to the lung are two in number, one for 
each of the pockets, in the external walls of which they are 
grooved, being merely, as it were, ploughed channels in the 
integument, which have been covered in by fibrous membrane. 
Each sends off several branches from its upper and lower 
edge, which respectively pass upwards and downwards, 
curving in their course, with their concavities facing each 
other, and terminate in the border of the pericardial gland. 
In the outer portion the vessels are, as I mentioned above, 
but passages in the integument (which here, from the particles 
of carbonate of lime imbedded in it, is white, as in the other 
regions of the body), but internally they lie between two 
transparent layers of membrane, and from this circumstance 
are easily observed in their passage to the pericardial gland. 
Each division or sac of the lung measures about 1 inch 
in length, and is a little more than a 1 inch from above 
downwards. The width is variable, depending, as it 
does, upon the condition of the body as to contraction or 
elongation. The course of the blood through the pulmonary 
vessels is more properly described under the head of— 
Circulatory System.—The course of the blood, in its passage 
through the bodies of molluscs, has long been misunderstood. 
Heretofore it has been thought that a perfect circulation ex- 
