134 ZOOLOGY. 



The rabbit shares the following features not only 

 with mammals, but with all reptiles (i.e. lizards, crocodiles, 

 turtles, and snakes) and birds 



(a) Absence of gills (not of gill-slits), at any stage in 



development ; 



(b) An amnion ; and 



(c) An allantois in development. 



The meaning of (b) and (c) we shall explain to the 

 student in the chapters on embryology. We simply mention 

 them here to render our table complete. 



The rabbit shares the following features with the 

 above animals and also with the amphibia (frogs and 

 newts), that is to say, with all the vertebra ta, except the 

 true fishes-*- 



(a) Lungs (but many fish have a swimming bladder 



which answers to the lungs in its anatomical 

 relations, and in certain fish this has the character- 

 istic pulmonary circulation) ; 



(b) Limbs which consist of a proximal joint of one 



bone, an intermediate part of two, and a distal 

 portion which has five digits, or is evidently a 

 reduced form of the five-digit limb ; 



(c) The absence of a median fin supported by fin- 



rays.* 



Finally the rabbit shares even with the true fishes (e.y. 

 sharks, dogfish, salmon, cod, and herring) 



(a) An epidermis consisting of more than one layer of 



cells ; 



(b) Skull, and vertebrae, replacing a notochord in 



development ; 



(c) A primitively three-vesicled brain, with paired 

 eyes, paired ears, and paired olfactory organs, all 

 of characteristic structure ; 



(d) A heart ventral to the alimentary canal, and a 



portal system ; 



(e) A large coelom, which does not extend anterior to 

 the heart, and from which a pericardia! chamber 

 is separated off. 

 * The frog's tadpole has a median tin, but no fin-rays. 





