BRANCH CHORDATA: CLASS REPTIL1A 313 



and snakes,* distinguished by the scaly covering of the 

 body, the Chelonia, which includes the tortoises and 

 turtles, distinguished by the shell of bony plates which 

 encloses the body, and the Crocodilia, which includes the 

 crocodiles and alligators, whose bodies are covered with 

 rows of sculptured bony scutes. 



Tortoises and turtles (Chelonia). TECHNICAL NOTE. 



Obtain specimens of some pond- or land-turtle common in the 

 vicinity of the school. The red-bellied and yellow-bellied terrapins 

 (Pseudemys] or the painted or mud-turtles (Chrysemys] are com- 

 mon over most of the United States. (Pseudemys is found south of 

 the Ohio River and Chrysemys north of it.) They may be raked 

 up from creek-bottoms or fished for with strong hook and line, 

 using meat as bait. They will live through the winter if kept in a 

 cool place, without food or special care of any kind. Observe their 

 swimming and diving, the retraction of head and limbs into the 

 shell, the use of the third eyelid (nictitating membrane), and the 

 swallowing of air. 



Examine the external structure of a dead specimen (kill by 

 thrusting a bit of cotton soaked with chloroform or ether into the 

 windpipe ; see opening just at base of tongue). Note shell consist- 

 ing of a dorsal plate, the carapace, and ventral plate, the plastron, 

 and the lateral uniting parts, the bridge. Note legs, and head with 

 horny beak but no teeth. Compare with snake. The examination 

 of the internal structure of the turtle can be readily made by saw- 

 ing through the bridge on either side and removing the plastron. 

 Note the ligaments which attach the plastron to the shoulder and 

 pelvic girdles. Note muscles covering these bones. Note just 

 behind the shoulder girdle the heart (perhaps still pulsating) and 

 the dark liver on each side of it. Work out the alimentary canal, 

 the trachea and lungs, and other principal organs, comparing them 

 with those of the snake. The skeleton can be studied by dissecting 

 and boiling and brushing away the flesh which still adheres to the 

 bones. The comparison of the skeleton of the turtle with that of 

 the snake is very instructive ; marked differences in the skeletons of 

 the two kinds of reptiles are obviously correlated with the differ- 

 ences in habits and shape of body. Note in the skeleton of the 

 turtle especially the shoulder and pelvic girdles and limbs (absent in 

 the snake) and small number of vertebrae and ribs. 



Among the common turtles and tortoises of the United 

 States are several species of soft-shelled turtles (Trio- 



* By many zoologists the lizards and snakes are held to form two distinct 

 orders, Lacertilia and Ophidia. 



