HOW ANIMALS BREATHE 



323 



spiracles, usually four in number, are the mouths of 

 sacs without the tubes, and the interior of the sac is 

 gathered into folds. Land snails have one spiracle, 

 or aperture, on the right side of the neck, leading to 

 a large cavity, or sac, lined with fine blood vessels. 

 These sacs represent the primitive idea of a lung, which 

 is but an infolding of the skin, divided up into cells, and 

 covered with capillary veins. 119 



FIG. 280. Part of a Transverse Section of a Pig's Bronchial Twig, x 240: a, outer 

 fibrous layer; b t muscular layer; c, inner fibrous layer; d, epithelial layer with cilia; 

 f, one of the neighboring alveoli. 



Like the alimentary canal, the lungs of an animal are 

 really an inflected portion of the outer surface ; so that 

 breathing by the skin and breathing by lungs are one in 

 principle. Indeed, in many animals, especially frogs, 

 respiration is carried on by both lungs and skin. 



In the course of embryonic development the lungs 

 of vertebrates are derived from the front part of 

 the alimentary canal. In some fishes, air is swal- 

 lowed, which passes the whole length of the diges- 

 tive tract, and is expelled from the anus. Here the 

 whole canal serves for respiration. In reptiles, birds, 



