The Lungs, Heart, and Blood-vessels of the Slug. 109 



have been unable, after the closest scrutiny, to detect any- 

 separate pulmonary vessel capable of conveying the blood to 

 the heart, and as I am well aware that the relations and use of 

 the would-be kidney have been much misunderstood. Ere we 

 proceed further I must show you how to expose the heart, 

 heart-gland, and blood-vessels, which we are about to enter 

 upon the study of. 



When dissecting the lung, you left untouched a sort of 

 island of skin and flesh, which was in the living animal sur- 

 rounded by integument. This square patch is a sort of house 

 with two stories, an upper and lower ; the former contains the 

 remnant of the shell, and is named the shell-bag ; the latter 

 contains the circulatory organs. Carefully, with your curved 

 scissors, remove the shell-bag and the adjacent loose tissue, 

 and you will see a ring of brown gill-like plates, and within 

 this ring the little conical heart of the slug, the whole being 

 covered in by a beautiful gossamer- like membrane, which is so 

 exquisitely transparent that you can watch all the movements 

 of the heart and vessels.* You will now observe that the 

 blood travels from the lungs towards the pericardial gland 

 (brown ring) in the directions shown in Fig. 1. 



Having been poured from the great pulmonary vein of each 

 side into the numerous branches of the lung network, and by 

 this means exposed fully to the action of the atmospheric air, 

 the blood flows in two principal directions, according to the 

 portions of the network which it has traversed. Thus that 

 which had passed downwards from the great veins, and had 

 descended to the bottom of the sacs, now ascends and reaches 

 the border of the pericardial gland, and that which had travelled 

 to the superior portion of the lung descends and meets that 

 which has journeyed upward, at the same plane (border of 

 pericardial gland). This process goes on in both lung sacs 

 simultaneously ; and we find that all the blood which has been 

 purified in the respiratory organ must flow to the border of this 

 gill-like gland, prior to its entrance into the heart. Next, the 

 blood flows in a perfectly centripetal manner through this 

 gland till it reaches its inner edge. This is hemmed round 

 by a transparent membrane, which in the posterior half consti- 

 tutes a kind of semi-canal, and in front expands in the form of 

 a sextant, as shown in the diagram (fig. 4). Into the folds, 

 then, of this double membrane (half canal, half bag) the blood 

 is poured. Now the narrow extremity of the sextant- shaped 

 bag opens directly into the base of the heart, there being a 

 small fold of membrane placed in the aperture, which plays the 



* If the animal be not dead the heart may be seen with tho greatest distinct- 

 ness, and its pulsations counted. It would be difficult to conceive of a prettier 

 object than the circulatory organ when in this condition. 



