148 Dr. T. Williams on the Mechanism of Aquatic 



in various parts of the pulmonary cavity in every species of 

 Limax^, 



If the internal and external coats of the vessels assume thus 

 the characters of two concentric non-calcified cylinders, it follows 

 that the mineralized layer must occupy the intervening space, 

 representing the middle coat. This is probably the truth. But 

 whether the external membrane and the lining of the cavity be 

 separate structures or not, it is indisputable that the vessels are 

 enveloped in a thick calcareous sheath. It is obvious, therefore, 

 that the respiratory gases respectively leaving and entering the 

 blood must traverse this dense septum, — a peculiarity quite with- 

 out a parallel in the whole history of the organs of respiration. 

 In the higher animals the pulmonary vessels in all classes are 

 brought so near the aerating surface, that anatomists of eminence 

 are not wanting who assert that such vessels are not covered by 

 any kind of epithelium whatever. If then all structures inter- 

 posed between the blood and the air are thus studiously reduced 

 to the utmost extreme of tenuity in the higher animals, how is 

 it that in these highly organized Gasteropods the respiratory 

 vessels should not only be encumbered by thick coats, but by 

 those of an inorganic nature ? 



Either in these animals respiration is reduced to a minimum, or 

 these calcareous coats not only do not interfere with, but literally 

 facilitate the interdifiusion of the gases. Although this idea 

 presupposes a mechanism quite unique in the history of the 

 breathing organs of animals, it involves nothing which contra- 

 dicts the laws of endosmosis. The mineral which is deposited 

 in the coats of the vessels is the carbonate of lime. It is im- 

 possible that such a substance could exert any chemical affinity 

 either for oxygen or carbonic acid. The lime-particles are so 

 loosely strewn together that they are separated by spacious in- 

 terstices. Such a septum would present no obstacle to the diflPu- 

 sion of gases. In fact the endosmotic conditions of the beautiful 

 experiment devised by Prof. Graham, in which he coated a coarse 

 sheet of brown paper with a thin film of collodion, are precisely 

 imitated in the vessels of the Slug. Prof. Graham found that 

 the organic film so placed did not in the least degree diminish 

 the rate of difiiision at which gases pass through a sheet of brown 

 paper. 



It is quite certain then that the vessels of the lung-sacs of the 

 Limacidse are encircled by the calcareous coats. The lime is not 



* I invite especial attention to this point, because hitherto all anatomists 

 have denied the existence of ciliated epithelium in the interior of the pul- 

 monary chamber of air-breathing Gasteropoda. I have proved the fact of 

 its presence beyond doubt, and by repeated observations. 



