The Lungs, Heart, and Blood-vessels of the Slug. 113 



Which three results are quite sufficient in themselves to 

 show that there is not much reliance to be placed in the 

 statements of those who assert that the gland is a kidney. 



Again, if this structure really fulfilled the office of an urine 

 gland it is evident that it should have attached to it some 

 channel through which the excrementitious fluid might be 

 carried out of the body. Even the opposite side admits this. 

 Here, however, we differ. Siebold, Cuvier, Muller, and others 

 maintain that there is a tube connected with the gland, and 

 opening near the lung aperture. I, on the other hand, 

 contend that no such canal is present. I have dissected many 

 specimens with the hope of discovering something of the kind, 

 but I have invariably failed to observe it ; and I can only 

 account for the mistaken observations of such distinguished 

 naturalists by assuming that in emaciated German and French 

 slugs the end of the intestine (which pursues exactly the 

 route of the supposed duct,) has been looked on as the canal 

 of the gland. 



The pericardial gland is of a dark reddish brown colour, 

 and measures from side to side (including the heart and sinus) 

 more than half an inch. It is made up of a great number of 

 lamellas or plates lying against each other, like the leaflets of a 

 fish's gill. Each of these examined under the microscope 

 appears to be composed of numerous irregular vacuoles, con- 

 taining within them solid, round, opaque, incompressible nuclei. 

 Passing between the plates may be observed hundreds of 

 blood vessels journeying from the lung to the sinus, and on 

 their passage giving off several branches, which wind about 

 the vacuoles and anastomose frequently. I cannot see that 

 there is any necessity for supposing that a slug has a kidney at 

 all. But even did I suppose the creature possessed of such a 

 commodity, I cannot conceive why I should pitch upon the 

 pericardial gland as the organ most likely to subserve the 

 function, simply because one of the compounds found in human 

 urine was discovered (or said to be discovered) here also. As 

 well might a person ignorant of anatomy, contend that the skin 

 of man was the human kidney, because there is undoubted 

 proof of the statement that urea (a constituent of the renal 

 secretion,) is thrown out from the body in the perspiratory 

 fluid. So long as scientific men frame generalizations upon 

 such vague and careless observations as those I have alluded 

 to, so long will comparative anatomy be uncertain and unre- 

 liable. If the reader will take the trouble to investigate the 

 matter for himself he will then ascertain how loosely many of 

 the conclusions of comparative anatomists have been formed, 

 and how important it is to accept no statements as truths until 

 they have been verified by personal research. 



