20 ELEMENTARY ZOOLOGY. 



in the class differ more among themselves than those in the other 

 classes. It includes snakes, lizards, turtles, and crocodiles. Their 

 bodies are covered by scales or plates, instead of hair or feathers. 

 They lay large eggs very much like those of birds. They always 

 breathe oxygen from the air, like birds and mammals, even though 

 they may live in water. The blood is not constantly warm as in 

 the birds and mammals. 



23. Amphibians. In outward appearance some of the members 

 of this class look like some of the reptiles. Like the reptiles they 

 are cold-blooded. They do not have the scales and plates in the skin, 

 but are of smooth and even slimy surface. They are especially 

 noteworthy from the fact that they begin 1 ife breathing oxygen 

 from the water by gills, as fishes do, and later in life lose their gills, 

 acquire lungs, and get their oxygen from the air. as do the higher 

 forms. Amphibians include the frogs, toads, and salamanders. 

 This is not a very important grovr natpre. but it is intensely 

 interesting to the student of zoolog} ause it seems to be a con- 

 necting link between the water-bre> ing and the air-breathing 

 animals. 



24. Fishes. Fishes are characterized by the fact that they breathe 

 by means of gills throughout life. The body is often scaly; the 

 appendages are fin-like, instead of jointed legs; the blood is cold and 

 the heart has only two chambers. They are beautifully adapted to 

 life in the water and are easily recognized, as a rule. 



25. Vertebrates and Invertebrates. All the animals of which we 

 have thus far spoken agree in certain particulars. They all have a 

 dorsal rod of cells a kind of internal supporting skeleton which, 

 in its most developed form, we call the vertebral column or "back- 

 bone." The nervous system is always dorsal to this rod. The heart 

 is ventral to the digestive tract and the blood has red corpuscles. 

 There are never more than two pairs of jointed appendages (legs, 

 arms, wings, etc.). They are all known as Vertebrates. All other 

 animals, except a few which seem intermediate in some respects, are 

 classed as Invertebrates, and agree in the following particulars: there 

 is no vertebral column; the nervous system is chiefly ventral to the 

 digestive tract ; the heart, when present, is dorsal ; and the blood 

 usually has only colorless cells ; there is no definite number, or arrange- 

 ment, of appendages. The principal divisions (phyla) of the inverte- 

 brates follow. 



26. Arthropods. This phylum embraces more kinds of animals 

 than all the rest of the animal kingdom put together. It includes 



