HOW ANIMALS BREATHE. 743 



of gills in the different groups of fishes. Their origin from the food- 

 tract is clearly shown in the lowest fish, the worm-like Amphioxus, 

 which has gills formed from a large, barrel-shaped pharynx pierced with 

 transverse slits through which the water is drawn by cilia (Fig. 2, 

 page 645 of the September, 1881, " Monthly "). This creature differs 

 from all other known vertebrates in the possession of cilia as the means 

 of renewing the water. 



The lamprey and hag, and the shark family, have the branchiae 

 in separate pouches developed from and connected with the pharynx ; 

 while most common fishes possess the familiar form. 



In the embryonic state, the shark has external gills long, filament- 

 ous appendages projecting through the gill-slits. But gills of this 

 nature have a more remarkable and enduring character in the amphibia. 

 In the toads, frogs, and most salamanders, these external gills give 

 place early in life to internal fish-like gills, which are, in turn, replaced 

 by lungs. But in certain species these external organs persist through- 

 out the life of the animal ; and the group is consequently known as 

 the Perennibranchiata. The Proteus, of Austrian caves, has three 

 pairs of scarlet fringes on each side of its neck. In the axolotl, of 

 Mexico, the six gills are somewhat arborescent ; while in the Meno- 

 branchus, found in the Uniced States, they are plume-like. The Siren 

 of the Carolina rice-swamps is another member of this group. 



1 



Fig. 2. A, Head op a Piked Dog-fish( 3pinax\ showing the transverse mouth on the under sur- 

 face of the head, and tne apertures of the gill-pouches. 15. diagam of the structure of the gill- 

 pouches: o, o, external apertures; i, i, apertures leading into the pharynx; s, s, gill-sacs, 

 containing the fixed gills. 



These plumose appendages upon the neck of the perennibranchiates 

 would seem, judging simply by their appearance, to be as truly a part 

 of the outer skin as any gills of the invertebrates. Indeed, such rela- 

 tion is much less apparent in the crabs and in the true snails. But, for 

 reasons lying deeper than mere appearance, the gills of amphibians 

 are classed with food-tract organs. However, this involves a nice dis- 

 tinction, since the food-tract and skin are fundamentally one. 



Aerial Organs of the Food-Tract. Dismissing now the water- 

 respiratory organs of the food-canal, we pass to the consideration of 

 its special air-breathing organs. 



The habit among fishes of swallowing air has already been men- 

 tioned. It is probable that in many fishes this air, or a portion of it 

 is simply passed through the gills, or perhaps is held in the oral cavity 

 to aerate more highly the water that bathes the gills. However this 



