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 certain 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 fcetal 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, 

 to act. 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 blood 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. 



