302 PRACTICAL LESSONS IN SCIENCE. 



circulatory organs are double. The lungs are capacious, but in- 

 stead of moving freely, as in the case of mammals, they are fixed 

 to the dorsal aspect of the body cavity. They communicate 

 freely with the cavities of the larger bones. 



The matter of the adaptation of animals to their surroundings 

 is perhaps more easily shown in the study of birds than in the 

 study of any other group. The feet, the bill, the legs, the wings, 

 the neck, the plumage, the whole appearance and habit of the 

 bird depend on whether it is a wader, walker or swimmer; a 

 scratcher, climber or a bird of prey. The idea is common every- 

 where, but in some way it seems more prominent among the 

 the birds. 



Birds are oviparous, the eggs having a large amount of yolk, 

 and requiring a high degree of heat for their development. In 

 the fertilized egg, the process of development begins before it is 

 laid ; the blastoderm is formed, the primitive groove and medul- 

 lary folds appear, then the cranial flexure, the digestive canal, the 

 amnion and allantois membranes, and soon the young bird is 

 ready to open the way to liberty from his stony cell. 



Birds are intelligent; they have beautiful plumage and musical 

 voices, and are interesting to man in many ways. No group of 

 animals yet studied is as intimately related to man; none other 

 more available for study than the birds. Study birds; note the 

 migratory birds, the time they arrive and depart; note the ones 

 that remain during the whole year, and those that only remain 

 during the winter. Note also those birds that pass through your 

 locality in their spring and autumn migrations, stopping only 

 for a few days. Notice the birds that nest in your region ; the 

 kinds of nests they build ; when they build them ; how many eggs 

 are laid, and what was the period of incubation. Make a careful 

 study of the breeding and nesting habits of some particular pair 

 of birds, making full notes. Study the legs, feet, wings, bill and 

 other parts of birds and compare them. Study the bones and 

 skeletons of birds, and compare with those of other animals. 

 Notice that the claws of perching birds close automatically on 

 the perch. Examine the arrangement of tendons in the legs. 



