Evolution of Animals 479 



(10) The heart consists of two ventricles complete, 

 and of two auricles with one row of ventricular valves. 



(11) The left aortic arch persists. 



(12) The red blood corpuscles are nucleated in the 

 marrow, spleen, etc., in other regions they lose the nuclei; 

 they are oval in the camel, circular in others, and bicon- 

 cave. 



(13) The lungs are paired, with trachea and cartilages, 

 also with bronchi. 



(14) The hypoglossal nerve is intracranial. 



(15) The kidneys are the metanephros and each has 

 one ureter. 



(16) All are viviparous except the Monotremata. 



(17) There is an amnion and an allantois. 



When one compares the above characters it appears that 

 between the Amphibia and Mammalia all are either common 

 and fundamentally alike, or that certain characters in Amphibia 

 could by progressive evolution be gradually changed into mam- 

 malian ones, except for the formation of the amnion. Com- 

 paring the Reptilia and Mammalia the last named character 

 is one of fundamental agreement along with others, while 

 five most important characters disagree. These are indicated 

 in Reptilia by an asterisk. 



Such would yield at least slight presumptive e^ddence in 

 favor of the Amphibia being the ancestral mammalian stock. 

 Further the tailed forms or urodela — not the mature tailless 

 Anura that have branched off along a line of condensing spe- 

 cialization — ^particularly claim our attention. If one think 

 of a salamander or an axolotl, one marked external difference, 

 as compared with ordinary mammals, is the real or apparent 

 absence of hair. But more careful study reveals the abundant 

 presence — especially at certain stages in the life history — of 

 many fine sense organs or side organs, with delicate hair end- 

 ings in each, that are disposed around the mouth and in longi- 

 tudinal lines along the body. These are in contact with nerve 

 endings from the vagus or hypoglossal, and correspond to 

 similar structures met with in cyclostomes, in a simple state 



