ZOOLOGY. 559 



system the bones and muscles make also a perfect appear- 

 ance. The skeleton is hard, complete, and full of air 

 instead of marrow ; the muscles are red and separated, 

 the movements free and complex. 



3395. The elevated condition of the motor system is 

 demonstrated in the limbs, which here appear in the 

 greatest diversity. There are only two pairs of limbs, 

 thoracic, and abdominal or sexual members. These two 

 are equivalent, so long only as the sexual and abdominal 

 cavity belonging to the trunk proper predominate, and 

 therefore preserve the equilibrium. But the limbs, which 

 have been given to serve the office of the trunk, are des- 

 tined for progression or swimming, and are simply ter- 

 restrial or aquatic members. 



3396. The higher limbs are thoracic respiratory mem- 

 bers, which are filled with air and clothed with tracheae, 

 thus exercising a function conformable to the thorax. 

 The thorax has an aerial character. The highest thoracic 

 limbs must be aerial members. 



3397. As the abdominal members move upon the 

 earth or in the water, so do the thoracic in the air. The 

 terrestrial limbs are feet, the aerial limbs, wings. 



3398. The wings are in the member-formation the 

 extremes of the thoracic limbs. It does not follow from 

 this that they are the noblest in rank, but only that they 

 are the uttermost unto which bodily motion can attain. 



3399. The winged animal is the Bird. 



3400. The Bird is an Insect with fleshy limbs. 



3401. The wings of Birds repeat the alary appendages 

 of Insects in the flesh. 



3402. A Bird's wing is a strange but very instructive 

 composition. It consists, namely, of a Reptile's foot and 

 an Insect's wings. 



3403. We saw how the branchiae of the Insect dried 

 up, separated from the feet, and being liberated as wings, 

 were permeated by tracheae. In the Bird these wings 

 have remained standing upon the feet and been converted 

 into feathers. 



3404. A feather is an Insect's wing. 



