THE TURTLE. 117 



this is the most porous portion of the shell, and here, 

 for some reason, the membranes are most easily sepa- 

 rated. Subsequent increase in the size of the air 

 space is due to the evaporation of water from the egg. 



Some birds migrate. Their lives depend upon 

 obtaining a sufficient supply of food at every season 

 of the year. When the food of a bird disappears upon 

 the approach of winter, as does the food of most 

 insect-eating birds, the bird is compelled to go to some 

 place where food can be obtained. Woodpeckers do 

 not necessarily migrate, for their food is obtained 

 from insects found in dead trees. These are quite 

 largely available all winter. Ducks and geese 

 must migrate from the north, for the water, to 

 which their habits of life adapt them, becomes frozen 

 over. 



There are usually considered about eighteen orders 

 of birds. 



THE TURTLE. 



1. Chloroform the turtle. Cut off its head. 



2. Examine the shell. The upper part is the 

 carapace. The lower part is the plastron. Of 

 how many plates is each composed? How are they 

 united ? 



3. Saw through the union of the carapace and 

 plastron? Cut the skin loose from the plastron, all 

 around, and remove the plastron. 



4. See the stomach. Trace the intestine to the 

 cloaca. 



5. Notice the heart in its pericardium. Usually 

 the heart will beat for an hour or two after the turtle 

 has been chloroformed and its head cut off. 



6. Trace the aorta. Does one branch, or two, per- 

 sist? If two, which is the larger? 



7. Look at the lungs. Is there a diaphragm? 

 How are the lungs held in place? 



