248 AN INTRODUCTION TO ZOOLOGY. 



The Mammalia agree with the Amphibia in having- 

 two condyles, and differ from all the other vertebrates 

 in having no nucleus in the red corpuscles of the 

 blood. The chief distinction, however, between them 

 and all other animals, is that from which they derive 

 their name, namely, that they suckle their young, 

 which are produced alive instead of being hatched 

 from eggs. 



Fishes. It will be easily understood from what has 

 been said in Part I., Chap. IV., that since the Amphi- 

 bians (which in their adult stage are comparatively 

 highly-developed animals) have a larval stage that is 

 like a fish, we therefore suppose that all Vertebrates 

 are descended from fish-like ancestors. This is ren- 

 dered still more certain by the fact that the higher 

 Vertebrates, though they liave not, like Amphibians, 

 a free fish-like larva, yet pass through a fish-like 

 stage in the development of the embryo. As al- 

 ready mentioned (p. 56), the developing chick ex- 

 hibits at an early stage the rudiments of gills ; and 

 the same is true of other groups at a corresponding 

 stage. 



The gills of fishes consist, in their most primitive 

 form, of holes passing from the throat and opening to 

 the exterior, each lined with tissue adapted for respira- 

 tory purposes, and supplied with fine capillaries in 

 which the blood gets filled with oxygen. The holes 

 are spoken of as the gill-clefts. A large looped blood- 

 vessel carries the blood to and from each ; these ves- 

 sels are spoken of as the vascular arches of the gills. 



