VERTEBRATA. 3815 
circulation is double, as in Mammals, starting from a four- 
chambered heart. Respiration is more complete than in 
other Vertebrates. The lungs are fixed, and communicate 
with air-sacs in various parts of the body, as along the ver- 
tebral column, and also with the interior of many bones, 
as the humerus and femur, which are usually hollow and 
marrowless.’” 
Both brain and cord are much larger rela- 
tively than in Reptiles; the cranium is larger in propor- 
tion to the face; and the parts are not situated in one 
plane, one behind the other. The cerebrum is round and 
smooth; and the cerebellum single-lobed. The ears re- 
semble those of Crocodiles; but the eyes are well devel- 
oped, and protected by three lids. They are placed on the 
sides of the head, and the pupil is always round. The 
sexes generally differ greatly in plumage, in some cases 
more widely than two distinct species. But the coloration 
of either sex of any one species is very constant. 
A. Aquatic Brrpos.—Specially organized for swimming ; 
the body flattened, and covered with water-proof clothing 
—feathers and down; the legs short (the knees being whol- 
ly withdrawn within the skin 
of the body), and set far apart 
and far back; the feet web- 
bed, and hind toe elevated or 
absent. The legs are always 
feathered to the heel at least. 
They are the only Birds whose 
neck is sometimes longer than 
the legs. 
1. Pygopodes, or Divers.— 
These lowest of the feath- 
ered tribe have very short 
wings and tail, and the legs _ @ a ) 
he peed os = Vesa Fia. Sc Gap Pennan- 
they are obliged, when on tii). Falkland Islands. 
