The Lungs, Heart, and Blood-vessels oj the Slug. 107 



2nd. Cut transversely from left to right through the front 

 channel of communication, and parallel to your second incision, 

 till you reach the remnant of the pulmonic aperture. 



By these four incisions you have laid open the lungs, which 

 present themselves, not as a ring,* surrounding the undivided 

 central portion (the heart and shell-bag), but as two distinct 

 pouches of oblong form. These pouches enclose between them 

 the heart, heart-gland, and the shell-bag, and are shut off be- 

 low, behind, and in front, by folds of thin lining membrane, 

 which pass from the inner surface of the general skin of the 

 body. Each division measures about half an inch in length, 

 and is somewhat more than a quarter of an inch deep ; the 

 width is inconstant, depending, as it does, upon the condition 

 of the body as to elongation or contraction. The walls of the 

 lungs are composed of the general skin, which has within it a 

 snow-white lining. This latter is the true respiratory surface. 

 Blood-vessels cannot be said to ramify in it, but it is literally 

 tunnelled by passages which interlace in the most intricate 

 manner, forming a network in which the blood is exposed to 

 the influence of the atmospheric air, introduced through the 

 lung-opening. The blood which circulates in the lungs is not 

 sent there directly from the heart (as is the case in man), but is 

 carried to these sacs by two great veins, one of which lies on 

 each side of the body, and, by the aid of numerous branches 

 which contribute to form it, conveys the vital fluid from the 

 abdominal cavity to the pulmonary networks. The course 

 which the blood takes in traversing the lungs we shall investi- 

 gate under the head of 



ORGANS OF CIRCULATION. 



In vertebrated animals the vessels through which the blood 

 travels, in " going its rounds/' are of three kinds : — 



1 st. Thick-coated vessels, which bring the blood from the 

 heart. 



2nd. Microscopic channels called capillaries,t which are 

 present in almost every tissue of the body. 



3rd. Thin-coated tubes with valves, which carry the blood 

 from the capillaries to the heart; these are called veins. 



Formerly, when the habit of drawing analogies between 

 vertebrates and invertebrates was more frequent than it is in 

 our day, it was supposed that in all gastropods what was 

 termed a complete circulation existed; that is to say, the blood 



* Von Siebold and others have stated that the lungs in this creature are of 

 an annular character. I leave the reader to judge between myself and so learned 

 an anatomist. 



t So called from " capilla," the Latin for a hair, hut they are very much 

 smaller than any hair, and clusters of them placed together would not be as thick 

 as a human one. 



