GENERAL PRINCIPLES 



21 



ing by means of gills or through the body; usually with two pairs 

 of well-developed antennae (feelers) ; and covered with a chitinous 

 or calcareous crust or shell. 2. Arachnids (e.g. Spiders and Scor- 

 pions) are land Arthropods breathing 

 by air-sacs; have four pairs of legs; 

 and no antennae. 3. Myriapods (e.g. 

 Centipedes) are land Arthropods with 

 numerous legs; one pair of antennae; 

 and no wings. 4. Insects (e.g. Grass- 

 hoppers and Butterflies) are also land 

 Arthropods with one pair of antennae, 

 but with three pairs of legs, and nearly 

 always with wings. 



IX. Vertebrates are eminently 

 characterized by the possession of a 

 vertebral column, which, in all but the 

 very low forms, is a thoroughly ossi- 

 fied backbone. Vertebrates include the highest known of all ani- 

 mals. 1. Ostracoderms (e.g. Armour-fishes, now wholly extinct) 

 are among the very simplest of Vertebrates (see Fig. 80). They 

 are of particular interest from the standpoint of the evolution of 

 the Fishes and higher Vertebrates. Characteristic features will be 



Fig. 13 

 modern chamber-shelled 

 Cephalopod (Nautilus) show- 

 ing the internal shell struc- 

 ture. 



Fig. 14 

 A modern Squid. (After J. H. Blake, from Shimer's "Introduction to the 

 Study of Fossils," permission of The Macmillan Company.) 



given beyond in connection with the life of the Devonian period. 

 2. Fishes always live in water; have distinct cartilaginous or bony 

 vertebral column; distinct jaws; pairs of fins; and gills. Sub- 

 classes of Fishes will be described later. 3. Amphibians (e.g. Frogs 

 and Salamanders) live either in water or on land. In the early 

 stage of development of the individual (e.g. Tadpole stage), they 

 are aquatic, and breathe by gills, while in the adult stage they 

 breathe by lungs and are largely terrestrial in habit. They never 



