VERTEBRATA. 
327 
them, that the 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. 161 
Subclass I.— Ratitae {Cursores). 
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 {DromcBus) of Australia, and Apteryx , or Kiwi- 
kiwi, of New Zealand. Besides these, there are extinct 
gigantic forms from Madagascar Rpyornis) and from 
