BKIEF REVIEW OF THE SYSTEM OF LIFE. 416 



Vertebrates. 



The more prominent characteristics of the six classes of Vertebrates are the fol- 

 lowing : — 



1. Mammals. 



Species that suckle their young ; breathe by means of lungs ; have a heart of four 

 cavities. There are three prominent subdivisions : (1) The true Viviparous, as Man, 

 ordinary Quadrupeds, Bats, Whales, Seals. (2) The Semi-oviparous, the young of which 

 are more immature at birth (the birth, therefore, intermediate between the oviparous and 

 viviparous), and which are passed into a pouch where they are suckled until maturity : as 

 the Marsupials, of which the Kangaroo of Australia and the Opossum of North America 

 are examples. (3) The Oviparous, or Monotremes, as the Ornithorhynchus of Australia 

 and Tasmania, and the Echidnte of Australia, Tasmania, and New Guinea, which produce 

 true eggs. The Ornithorhynchus, or Duckbill, has the bill of the Duck, and lives along 

 streams in holes entered below the level of the water. It has the bones that in Marsupials 

 support the pouch, but not the pouch. 



2. Birds. 



Oviparous ; breathing by lungs ; heart of four cavities ; covered with feathers, and 

 having wings mostly adapted for flying. All existing birds have bills without teeth ; but 

 geological discovery has made known the existence in Mesozoic time of birds with a full 

 set of teeth. 



3. Reptiles. 



Oviparous; breathing by lungs ; a heart of three or four cavities ; naked or covered 

 with scales : as Crocodiles, Lizards, Turtles, Snakes. 



4. Amphibians. 



Oviparous ; breathing when young by gills, afterward by both gills and lungs, or by 

 lungs alone ; a heart of three cavities ; naked or covered with scales : as Frogs, which lose 

 the tail as well as gills on becoming adults ; and Salamanders, the tailed (or lizard-like) 

 Amphibians. The modem species are naked-skinned and often toothless ; but many 

 ancient kinds had scales like Reptiles and stout teeth. 



5. Fishes. 



Usually oviparous ; heart usually of two cavities ; breathing by gills, which take air 

 from the water, and are situated in front of one or more openings in the sides of the throat 

 that let out water which enters by the mouth ; skin naked, or covered with scales or bony 

 plates. Locomotion chiefly a process of sculling by means of the posterior or caudal 

 extremity of the body. 



Under Fishes there are the following prominent divisions : — 



Pal^ichthtes, or Fishes of ancient type, including the Sharks and Gars, charac- 

 terized by a heart with the arterial bulb contractile and the intestine having a valve 

 between it and the stomach, both characters showing relations to the Amphibians. The 

 three grand divisions are : — 



1. Selachians. — The group includes the Sharks (Fig. 355) and Rays — Fishes having 

 a cartilaginous skeleton ; usually several gill openings or slits (g) ; no gill-cover, and gills 

 attached to the skin by the outer margin instead of being free ; embryo with deciduous 

 external gills ; no air-bladder ; usually no proper scales, but a rough skin (shagreen) . 

 The ordinary Sharks have the mouth underneath and separate from the nostrils, with the 

 teeth sharp-edged (Figs. 358, 359, 360) ; another group, the Hyhodonts, have the teeth of 



