2 58 COMPARATIVE MORPHOLOGY OF VERTEBRATES, 
a series of sacs or infundibula lined with alveoli. The infundibula 
open into a central chamber, which, since it is ciliated and has numerous 
glands in its walls, may be compared toa bronchiole. In the toads and 
aglossa the alveoli are more extensively developed in correlation with 
the more terrestrial habits. 
Tt has recently been shown that a number of terrestrial urodeles are lungless in 
all stages of development, and that no traces of larynx or trachea occur, even after 
the gills are absorbed. In these species there is a great development of capillaries 
in the skin and in the walls of the mouth and pharynx, the respiratory functions 
being transferred to these parts. In the frogs the skin is also respiratory and it is 
largely supplied by the cutaneous arteries which arise from the same arch as the 
pulmonary arteries. 
In the amphibia the air ducts enter the anterior end of the lungs, 
but in the amniotes the lungs extend anteriorly to the entrance of the 
bronchi which is on the medial side. This change is in part the result 
of the transfer of the heart into the thorax, the position of the pulmonary 
arteries forcing the bronchi toward the centre of the lungs. In the 
amniotes, also, the ducts are characterized by the presence of cartilage 
in their walls, so that they are true bronchi. These bronchi may also 
extend inside of the lungs, often dividing into secondary and tertiary 
bronchi inside them. 
REPTILES.—In many reptiles (snakes, amphisbenans, many 
skinks) the lungs are asymmetrical (left usually larger in snakes, right in 
lizards) and exceptionally one may be absent in snakes. The internal 
structure shows considerable variation. The simplest conditions are 
found in the snakes and in Sphenodon (fig. 267), where the lungs consist 
of a single sac lined with infundibula in the basal portion (snakes) or 
throughout (Sphenodon). In the lizards (fig. 268) one or more par- 
titions or septa extend from the distal wall of the lung nearly to the en- 
trance of the bronchus, thus dividing the lung into chambers lined with 
alveoli; while a part of the bronchus may extend (main bronchus, 
fig. 268, B) to’the extremity of the lung. In the chameleons the septa 
do not reach the distal wall so that the chambers communicate here as 
well as at the proximal side, the result being that the bronchus enters a 
cavity, the atrium, which connects with the chambers separated by the 
septa, and these in turn open into a terminal vesicle, a condition recall- 
ing the parabronchi of the birds, soon to be described. This resem- 
blance is heightened by the development in these same lizards of long, 
thin-walled sacs from the posterior part of the lung which extend among 
