254 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
In the.whales and young marsupials the larynx is prolonged so that it projects 
into the choana behind the soft palate. In the whales (fig. 262) this is an adaptation 
to the manner of taking food from the water and breathing at the same time. In 
the young marsupials the milk is forced into the mouth by the muscles of the mam- 
me of the mother and this arrangement prevents strangulation. 
Trachea.—In the tetrapoda the trachea is strengthened by the 
formation of cartilage in its walls, the beginnings of which are seen 
in the urodeles where the fifth branchial arch gives rise to these ele- 
ments (p. 251). Their arrangement varies considerably in the urodeles 
_and cecilians, being sometimes scattered pieces, 
sometimes regularly arranged and even united in 
the lateral walls (fig. 258). Corresponding to the 
posterior position of the lungs the trachea is long 
in these groups, but in the anura it can scarcely 
be said to exist, the lungs succeeding almost 
immediately to the larynx. 
In the reptiles the trachea varies in length, 
being shortest in lizards (except amphisbznas), 
longer in snakes, tortoises and crocodiles, divid- 
Fic. 262.—Larynx of ing into bronchi at varying distances from the 
peas a faltes lungs. It is frequently bentin turtles. In many 
showing the prolongation reptiles the cartilage rings of the trachea are in- 
Po ee complete, but in Sphenodon, lizards and some 
eae cricoid; snakes some cartilages (usually the more anter- 
' ior) form complete rings, the others being com- 
pleted dorsally by membrane. In snakes the successive rings are 
often united, especially on the sides. 
The trachea is greatly elongate in birds in correlation with the 
length of the neck and the position of the lungs within the thorax. 
The rings, which are usually complete, are frequently ossified. The 
trachea is occasionally (male ducks, etc.) widened in the middle and 
in various groups becomes greatly convoluted so that its length from the 
glottis to the lungs exceeds that of the neck. In some these convolu- 
tions occur beneath the integument of the thorax; in some between the 
sternum and the muscles; and in the cranes and swans within the 
keel of the sternum. 
The larynx is never the organ of voice in the birds, its place being 
taken by a‘somewhat similar structure, the syrinx, at the division of 
the trachea into the bronchi. The sound-producing elements are 
