Respiratory System. 173 



have the double organs -a lung cavity and a >/\\\ cavitv, which arc used 

 alternately or as required. In the case of those mollusks that live 

 chiefly on land and breathe exclusively air, viz., the various Slugs, pond 

 and land Snails, and the operculated C} T clostomi, &c., the respiratory 

 cavity occupies the same position as in the aquatic and amphibian spe- 

 cies, but the modification of its structure has now turned it into a true 

 pulmonary chamber or lung. 



There are several species of Crabs that inhabit' the land, partly or ex- 

 clusive]}', whose gill cavities are all more or less modified for breathing 

 air. The gill cavity of the land Palm Crab ( Birgus-latro ) is divided 

 into two, the upper one, a true lung, never containing anything but air, 

 which there comes into contact with both veins and arteries. The pond 

 snail ( Limnea ) proves, in its own development, the modification of gill 

 cavity into lung, because when it is first hatched, for a time it breathes 

 only water, but afterwards it changes to air breathing exclusively. 

 Prof. Forel, however, found Limneans in 130 fathoms of water in Lake 

 Geneva, that, of course, breathed only water all their lives, but on be- 

 ing brought to the surface they filled their respiratory cavities with air, 

 and discarded the water breathing. This shows how nearly alike water 

 breathing and air breathing may be after all. At any rate, it proves 

 how easily air breathing ma} r succeed water breathing by the same ap- 

 paratus on the principle of a greater including a less, since if the ap- 

 paratus could get sufficient free oxygen from the water, which contains 

 but little, it need not be astonishing that it could get it from air which 

 contains much. 



There are several genera of fishes belonging to several different fam- 

 ilies, that possess the double breathing apparatus. These fish are called 

 Labyrinthici, a term suggested by the peculiarity of their gill cavities. 

 These cavities are very large, and are only parti}' filled by the gills, the 

 rest of the space being occupied by folds and doublings of the mucous 

 membrane more or less complicated. These folds increase the surface 

 of membrane that is exposed to the action of the air which is admitted 

 to the cavity, and which there, by endosmose, oxydizes the blood on 

 the opposite side of the membrane. It is plain how fishes provided 

 with this air-breathing apparatus might be able to live out of water, and 

 so we find several kinds that spend much of their time on land, and 

 sonic make long journeys across the country, using the spines on their 

 tins and gill covers as organs of locomotion. There are two genera of 

 of the (Joby family notable for their partiality to land life. It is a mat- 

 ter of conjecture not merely, but of plain certaint}-, that if left to them- 

 selves for some ages the descendants of the Gobies would be represented 

 by animals having tolerable sort of feet attached to their gill covers and 

 side fins, and well developed lungs at the sides of the neck. But the 



