264 COMPARATIVE -MORPHOLOGY OF VERTEBRATES. 
the hinder part of the alimentary tract is also respiratory. ‘Thus in 
Cobitis water is drawn in and expelled from the anus, and the posterior 
half of the digestive canal is richly vascular and is the seat of consider- 
able respiration. 
Before hatching or birth the lungs of the amniotes are unable to 
function, while a certaim amount of oxygen is necessary for the devel- 
opment and the carbon dioxide formed must be carried away. This 
respiratory function is assumed by the allantois. The allantois is 
a ventral diverticulum from the hinder part of the alimentary canal, 
which during foetal or embryonic life, acquires a relatively enormous 
development. It extends beyond the body limits and ‘in reptiles and 
birds comes into close relations with the porous egg shell, while in the 
mammals it plays an important part in the formation of the placenta. 
In all these the allantois is extremely vascular, developing a rich net- 
work of blood-vessels close to the shell (sauropsida and monotremes) 
or to the walls of the maternal uterus, (mammals) which serves for 
the rather limited exchange of gases necessary for the young. After 
free life begins the allantois is either absorbed (sauropsida) or is lost with 
the rest of the placenta (mammals), only the basal part persisting as the 
urinary bladder, described in connection with the urogenital system. 
ORGANS OF CIRCULATION. 
The functions of the circulation are two-fold: to carry food and 
oxygen to the tissues and organs of the body and to remove the waste 
from them. In addition it has been made probable that every activity 
of the body results in the formation of peculiar substances—activators— 
which have fixed and definite effects upon the various organs. These 
activators pass into the blood and form the stimulus which may cause 
other organs or cells, remote from the place where the activator is formed, 
toact. This subject is a new one and much may be expected from it in 
the future. 
The structures concerned in the circulation are two fluids, the blood 
and the lymph; and the vessels (vascular system) in which the fluids 
circulate, certain parts of the vessels being specialized (hearts) for the 
propulsion of the blogd and lymph. A blood heart occurs in all verte- 
brates in connexion with the blood circulation; most vertebrates have 
lymph hearts in connexion with the lymph vessels, but in the higher 
groups the flow of the lymph is due to the blood pressure and also to the 
motion of the parts through which the lymph vessels course. 
