192 THE ANIMALS AND MAN 



nous) spinal column is the characteristic by which we dis- 

 tinguish them from the invertebrate or backboneless animals. 

 Furthermore, all of the vertebrates possess an internal 

 skeleton which is in most cases composed of bone, and is 

 firm and strong. In some of the lower fishes, as the sharks 

 and sturgeons, the skeleton is made up of cartilage, tough 

 but not hard. The vertebrate skeleton consists typically 

 of an axial portion comprising the spinal column and head, 

 and of two pairs of appendages or limbs, variously developed 

 as fins, wings, legs and arms. In some vertebrates these 

 limbs are represented by mere rudiments, and in the lowest 

 fish-like forms, the lancelets and lampreys, there is not 

 the slightest trace of limbs. A part of the central ner- 

 vous system, the spinal cord, runs longitudinally through 

 the body on the dorsal side of the alimentary canal; the 

 circulatory system is closed, the blood being always confined 

 in the heart and in vessels called arteries, veins, and capil- 

 laries, and the blood is red in color owing to the presence of 

 numerous red corpuscles or blood-cells. The nervous 

 system is highly developed, with a large brain in all the 

 typical forms, and with complex and usually highly efficient 

 special sense-organss. Respiration is carried on by means 

 of external gills, or by internal lungs which communciate 

 with the outside through the mouth and nostrils. To the 

 lungs and gills the blood is brought to be "purified," i.e., to 

 give up its carbon dioxide and to take up oxygen. 



Classification.- -The Chordata are variously divided by 

 zoologists into eight or ten classes, of which (in the eight- 

 class system) the five classes* Pisces (fishes), Batrachia 

 (batrachians), Reptilia (reptiles), Aves (birds), and Mam- 

 malia (mammals), belong to the true vertebrates. These 

 classes will be considered in the following chapters. 



*The animals included by some zoologists in the single class Pisces, are 

 held by other zoologists to constitute three distinct classes, thus making a 

 subdivision of the branch into ten classes. 



