2 70 COMl'AKATXVE MORPHOLOGY OF VERTEBR^\TES 



lung (fig. 282). 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. 



The Air Ducts 



The opening from the pharynx into the air ducts is known as the 



t/ glottis, usually an elongate slit capable of being closed and opened 



by appropriate muscles. This is immediately succeeded by the 



ducts, which, except in the dipnoi, are more or less differentiated into 



regions and have skeletal supports in their walls. 



In the dipnoi the glottis is either in the mid-ventral line (Protopterus) or a 

 little to one side (Lepidosiren, Ceratodus) and the air duct passes up on the right 

 side of the oesophagus to reach the lungs which are dorsal to the alimentary 

 canal (fig. 290). The tube is without skeletal supports and connects directly 

 with both lungs without any division into bronchi. 



Larynx. — The beginnings of the larynx are seen in the amphibia, 

 where in the lower types {Necturus) a pair of cartilages are developed 

 on the sides of the glottis, in the position of a reduced visceral arch, 

 each cartilage extending posteriorly a short distance along the air 

 ducts. In other genera of urodeles the anterior end of each lateral 

 cartilage separates from the rest as an arjrtenoid, the first of the 

 laryngeal cartilages, imbedded in the walls of the glottis. The rest 

 of the lateral cartilages may remain entire (fig. 291), or they may 

 separate into a number of pieces, extending along the lateral walls of 

 the trachea and bronchi. Usually the anterior pair of these pieces 

 fuse in the mid-ventral line, thus forming the second (cricoid) ele- 

 ment of the pharyngeal framework. These parts are removed by 

 antagonistic muscles. One set of these, extending to the persistent 



