3^4 ELEMENTARY ZOOLOGY 



are not bony, but are true skin structures. Note the modi- 

 fication of the scales on the head, back, and ventral sur- 

 face. Those on the dorsal surface often have minute 

 ridges, the keels. How do the ventral scales differ from the 

 dorsal ones and others ? By a system of muscles these 

 ventral scales are rhythmically moved and as their posterior 

 edges are pushed back against some resisting object the 

 body glides forward. On the head note the pair of eyes. 

 Are there eyelids ? In front of each eye note an opening. 

 What are these openings ? Thrust a bristle into the 

 opening and see where it enters the mouth-cavity through 

 the internal nares. Does the snake have external ears ? 

 Observe the very long jaws and note that they are loosely 

 hinged. Examine the inside of the mouth. Are there 

 teeth? If so where are they situated, and how arranged? 

 Note that all of the teeth point backwards. Food is not 

 chewed. When some object of prey, a frog, or mouse, 

 for example, is seized, the teeth hold it fast to the roof of 

 the mouth and by a backward and forward movement of 

 the lower jaws it is gradually drawn into the large 

 oesophagus. What is the character and situation of the 

 tongue f Just behind the tongue note the narrow slit, 

 glottis, opening into the tvindpipc, or trachea. Back o/ 

 the trachea opens the oesophagus. 



When the snake is laid open the elongate heart will 

 be conspicuous in the anterior third of the body. Insert 

 a blowpipe or quill into the glottis just back of the tongue, 

 and inflate the lung, which is a long, thin- walled bag 

 extending from the region of the heart posteriorly for 

 two-thirds of the length of the body. There is but one 

 developed lung, the right ; note at the anterior end of the 

 lung a small mass of tissue, the atrophied left lung. 

 Running forward from the lung is a long tube composed 

 of incomplete cartilaginous rings, connected by mem- 

 brane, the trachea. Note the long straight alimentary 



