THE ANIMAL CLIMAX-THE CHORDATES 



331 



the loose skin at the base of the fangs 

 causes the poison to be inoculated into the 

 wounds. Once the ^'enom has entered the 

 blood stream, its effect is very rapid, caus- 

 ing a small rodent to become paralyzed in 

 a matter of a few seconds. The venom acts 

 by destroying the red blood cells. Anti- 

 venoms ha\'e been prepared for all of the 

 common poisonous snakes and have proved 

 valuable in alleviating the effects of their 

 bite. A normal person rarely dies from the 

 bite of a poisonous snake, although he may 

 be very sick. Of the four types of poisonous 

 snakes in the United States, the most com- 

 mon are the rattlesnakes; the others are the 

 water moccasin (cotton mouth), the cop- 

 perhead (Fig. 13-51) and the beautifully 

 banded coral snake. The last, although poi- 

 sonous, is quite different from the other 

 three in that it has no fangs but resembles 

 the Gila monster in its method of imparting 

 venom. 



AIR CONQUERORS: THE BIRDS 



It is a far cry from the lowly tiu'tle and 

 slithering snake to the most decorated of all 

 animals, the birds, yet with few exceptions 

 the birds are little different from the rep- 

 tiles (Fig. 13-52). An early biologist has 

 called them "glorified reptiles," which de- 

 scribes them very well. They have mastered 

 the air, as is obvious to the hunter who 

 matches his marksman's skill against the 

 abilities of the wary mallard to avoid being 

 hit. In conquering the air several body 

 modifications have been necessary. The 

 drastic need for a lighter body framework 

 was met by lighter, hollow bones, and a 

 lessening of the weight of the outer cover- 

 ing (Fig. 13-53). 



The scales of the reptiles have probably 

 given rise to the feathers of birds, and 

 actually there is little difference between 

 them except in weight and textiu-e. 

 Their general arrangement is very similar. 

 The bird still carries scales on its legs and 

 sometimes around the base of the beak. Be- 



Fig. 13-51. The copperhead {Agkistrodon mokasen) is 

 one of our venomous snakes. Note the vertically slit 

 pupil and the "pit" just in front of the eye. These 

 two features characterize most poisonous snakes. 



cause of their loose arrangement, feathers 

 act as excellent insulating material, an ex- 

 tremely important need for birds who often 

 fly in regions of very low temperatures. The 

 wings are the modified anterior legs of the 

 reptiles in which the three fingers have 

 fused at the tip and the space between has 

 been spanned by a sheet of skin covered 

 with feathers. The wing spread is due prin- 

 cipally to the large quill feathers at the 

 outer edges of the wings. The breast bone 

 is greatly over-developed. It is known as a 

 keel and functions as an anchor for the 

 powerful breast muscles which are used to 

 give the power stroke in flight. 



Other structural modifications have been 

 essential for the flying animal. In order to 

 keep active during all times of the year in 

 temperate and arctic regions, it was neces- 

 sary to maintain the body at a constant 

 temperature which, in birds, is slightly 

 higher than in man. This, of course, meant 

 greater expenditure of energy, thus requir- 

 ing a better circulatory system. Many of the 

 higher reptiles possess four-chambered 

 hearts (alligators and crocodiles), whereas 

 among birds it is the universal rule. The 

 hearts of birds are large and very well 

 developed, an essential factor in maintain- 

 ing a rapid circulation when so much 

 energy is consumed in flight. 



