260 .ELEMENTARY ZOOLOGY 



vertebrates. The possession of a backbone or bony 

 (sometimes cartilaginous) spinal column is the character- 

 istic by which we distinguish them from the invertebrate 

 or backboneless animals. Furthermore, all of the verte- 

 brates 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 append- 

 ages or limbs, variously developed as fins, wings, legs 

 and arms. In some vertebrates these limbs are repre- 

 sented 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 nervous 

 system, the spinal cord, runs longitudinally through the 

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

 culatory 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-organs. Respiration is carried on 

 by means of external gills, or by internal lungs which 

 communicate 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 carbonic-acid gas 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), 



* 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. 



