Y E RT K BRAT A. 327 
them, that tlie generative products and the refuse of di¬ 
gestion are all discharged through one common outlet. 
The sole organs of prehension are the beak and feet. 
The 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 com¬ 
municate with air-sacs in various parts of the body, as 
along the vertebral column, and also with the interior of 
many bones, as the humerus and femur, which are usu¬ 
ally hollow and marrowless. 168 Both brain and cord are 
much larger relatively than in Reptiles; the cranium is 
larger in proportion to the face ; and the parts of the brain 
are not situated in one plane, one behind the other. The 
cerebrum is round and smooth, and the cerebellum single- 
lobed. The ears resemble those of Crocodiles; but the 
eyes are well developed, and protected by three lids. They 
are placed on the sides of the head, and the pupil is al¬ 
ways round. The sexes generally differ greatly in plu¬ 
mage, in some cases more widely than two distinct species, 
but the coloration of either sex of any one species is very 
constant. 
There are two subclasses. 169 
Subclass I. —Ratitoe (Cur sores ). 
This small and singular group is characterized by hav¬ 
ing no keel on the breastbone, rudimentary wings, feath¬ 
ers with disconnected barbs, and stout legs. The African 
Ostrich has two toes, the Cassowary three, and the Apte¬ 
ryx four. 
Its representatives are the Ostrich ( Struthio ) of Africa 
and Arabia, South American Ostrich (Rhea), Cassowary 
( Casuarius) of the East Indian Archipelago and Austra¬ 
lia, Emu ( Dromceus ) of Australia, and Ajpteryx , or Kiwi- 
kiwi, of New Zealand. Besides these, there are extinct 
gigantic forms from Madagascar (AEpyornis) and from 
