278 PRINCIPLES OF ANIMAL BIOLOGY 



hard plates. They are cold-blooded in common with the fishes and 

 amphibia but unlike the following two classes. They have no gills in any 

 stage. The heart is three-chambered (approximately four-chambered in 

 crocodiles, in which the ventricle is partially divided). Some snakes are 

 poisonous, but most of them are beneficial to man (as are also the lizards) 

 because they devour noxious animals. Some turtles are used for food. 



The birds are characterized by feathers, which grow from pits in the 

 skin, forelimbs adapted to flight, a four-chambered heart, warm blood 

 (warmer than that of the next class, mammals), and a beak with horny 

 covering but no teeth. The bones of the skeleton are extensively fused, 

 particularly in the wings. The body is made light for its size by large 

 air spaces, variously placed, some of them extending into the cavities of 

 certain bones. These spaces connect with the lungs, but their walls are 

 not made of lung tissue, though doubtless they do effect some exchange of 

 oxygen and carbon dioxide. 



Mammals are mostly quadrupeds. The skin is covered with hair 

 — very sparsely in some. They breathe air even when they inhabit water. 

 The heart is four-chambered, the blood warm. The red cells of the blood 

 are devoid of nuclei except while they develop in the marrow. There is 

 a muscular sheet or diaphragm between the thorax and the abdomen, 

 important in breathing. The young are usually developed in the uterus 

 of the female — a few lay eggs — and are nourished with milk from the 

 mammary glands after birth. The most primitive mammals, the egg 

 layers, live in Australia and neighboring islands. The marsupials, which 

 give birth to their young in a very early stage and carry them for a long 

 time in a pouch, are next most primitive. They live in the Australian 

 region, in South America, and one kind (opossum) in North America. 



SUBPHYLUM I. Enteropneusta. Wormlike chordates of somewhat doubtful 

 systematic position. (Fig. 246.) 



Order 1. Balanoglossida Order 2. Cephalodiscida 



SUBPHYLUM II. TUNICATA. Saclike marine animals with a cuticular outer covering 

 known as a tunic or test. Tunicates. (Fig. 247.) 



Order 1. Ascidiacea Order 3. Larvacea 



Order 2. Thaliacea 



SUBPHYLUM III. Cephalochorda. Fishlikc chordates with a permanent noto- 

 chord composed of vacuolated cells. Amphioxus. (Fig. 248.) 



SUBPHYLUM IV. VertebratA, Chordates in which the notocliord cither persists 

 or becomes invested by cartilage, segmented, to form a vertebral colunui, or disap- 

 pears, the vertebral column being made up of bony segments. 



Class I. Cyclostomata. l']ellike vertebrates without functional jaws or lateral 

 appendages. Lampreys and hagfishes. (Figs. 249, 250.) 



Subclass I. Myxinoidea Subclass II. Petromyzontia 



