250 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
secretion and possibly of its absorption, the probability being increased 
by the greater abundance of the spots in species with closed ducts. 
While the pneumatic duct usually connects with the dorsal side of the alimentary 
canal, it enters the left side in Erythrinus, and in the mid-ventral line in Polypterus 
and in Calamoichthys. In Polypterus the bladder arises from the ventral side and 
there are paired swim bladders, the right being the longer. The blood in this genus 
comes from the efferent branchial arteries and hence is arterial. 
The swim bladder is supposed to have hydrostatic functions, aiding 
in the recognition of differences of pressure due to changes in depth. 
In the clupeids the air bladder sends a diverticulum into the head, 
there giving a branch to each ear. In some physostomes (siluroids, 
cyprinids, gymnonoti) parts of the anterior vertebre are modified into a 
chain of bones—the Weberian apparatus—adapted to convey dif- 
ferences of bladder pressure to the internal ears. One pair of bones 
is connected with the dorsal wall of the air bladder, a second with a 
diverticulum (sinus impar) of the internal ear, while others are in- 
tercalated between these extremes (fig. 257). Changes in the distention 
of the bladder are thus conveyed to the inner ear and probably affect 
the sense organs. 
LUNGS AND AIR DUCTS. 
Lungs arise as a diverticulum from the ventral side of the pharynx, 
immediately behind the last gill pouch. The diverticulum divides 
almost as soon as outlined into right and left halves, each the anlage of 
the corresponding lung. As development proceeds, the two grow in’a 
caudal direction into the trunk, carrying the peritoneum with them as 
they protrude into the ccelom, so that they eventually have an entodermal 
lining,. derived from the epithelium of the pharynx; an outer serous 
layer of peritoneum, with mesenchyme carrying blood- and lymph- 
vessels, nerve and smooth-muscle fibres between the two. In this 
development two parts are differentiated, the lungs, the actual seat of 
the exchange of gases, and the air ducts leading from the pharynx to 
them. The ducts may consist of an anterior unpaired portion, the 
wind-pipe or trachea, connecting with the pharynx, and usually divid- 
ing at its lower or posterior end into two tubes, the bronchi, leading to 
the two lungs. In most air-breathing vertebrates the anterior part of 
the trachea is specialized and forms a larynx. In addition to these 
parts, the mechanism by which air is drawn into and expelled from the 
lungs forms a part of the respiratory apparatus. 
