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 vertebrae 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 jear 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 coelom, 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. 



