■n-2 



iini) (I ai.],i:kv. 



() 



The intestinal Ccanal terminates iu a c/oarii or cavity tliruui;h which 

 the i)rotlucts of tlie uro-genital organs also pass. 



Trurhcn or U'iiuljjipe. — Finally, as an important anatomical peculiarity 

 of this class of Vertebrates, wc liave to mention that the trachea or 

 windpipe is composed of a scries of entire osseous rings. The organ 

 f voice is n(jt the larynx as in mammals, but is formed by a peculiar 

 modification of the lower end of the windpipe called the syrinx. The 

 syrinx may be formed either by the trachea or by the bronchi oulj', but 

 most commonly the lowest rings of the trachea as well as bronchi 

 participate in its formation. The modifications of the voice or song of 

 a bird are regulated by a \mY of "extrinsic" and, in the Song- 

 JSirds, several pairs of " intrinsic " muscles. The former, possessed 

 by all birds, generally pass from the trachea to the sternum and 

 pirciila. The intrinsic muscles may be absent or represented by five 

 or seven pairs. These differences afford important characters for the 

 purpose of classifying certain orders of Birds [rf. p. 107). 



Nest unit Eyys. — Birds arc, without exception, oviparous. The 

 majority deposit their eggs in a nest which they specially prejiare for 

 their reception. Incubation lasts for a shorter or longer period and 

 varies from 1 1 to 5(5 days. 



The eggs are on the whole fewer in number than is the case in 

 Reptiles; they possess a large amount of yolk and arc invested with 

 a hard porous calcareous shell. 



Youny. — The J'oung when hatched ditfer gieatly iu the relative 

 degree of development which they have attained. In the most primi- 

 tive condition the nestling emerges from the shell clothed in down and 

 capable of considerable activity, Ijut in the most specialized it is blind, 

 naked and helpless when hatched, and requires to be fed and cared for 

 by its parents for some time. 



Meittiil Fdcidties. — With regard to their mental faculties. Birds as a 

 class seem to occupy a position intermediate between Mammals and 

 Beptiles. Intelligence of a high order manifests itself in their social 

 relations with one another and in their various methods of obtaining 

 food. These faculties are still more developed in individuals which 

 come iu contact or live with man. 



Miyruthiii . — The ditticulty or impossibility of obtaining food when 

 the cold of winter destroys insect-life, or snow hides seed or other 

 vegetable nourishment, compels most birds to leave the locality where 

 they breed. Those which are stationary or range over only a limited 

 extent of country in search of food ai'e termed, resident Inrds. Their 

 movements are of an uncertain, erratic nature, and depend on external 

 and atmospheric conditions. But others, as soon as food becomes 

 scarce, following a common impulse, luigi'atc at lixed times and by 



