THE AIR-BLADDER AND LUNGS 281 



somewhat similar organ ("red-body") is present, but consists of 

 capillaries only. 



The air-bladder lies above the peritoneum on the dorsal side of 

 the body-cavity, between the vertebral column, aorta, and kidneys 

 on the one hand, and the alimentary canal on the other : it is 

 invested by the peritoneum on the ventral side only. It is more 

 or less sac-like in form, and is 

 only exceptionally paired (Poly- 

 pterus) ; it usually extends along 

 the whole length of the body- 

 cavity, and its walls are composed 

 of connective, elastic,and muscular 

 tissue. In some Teleostei it is 

 transversely constricted so as to 

 form several successive divisions ; 

 in other cases it may give rise to 

 a more or less numerous series 

 of CEFical processes.! Its internal 

 surface may be either smooth or 

 spongy (Fig. 225) owing to the 

 formation of a meshwork of 

 trabeculse, the structure of which 

 resembles that of the lungs of 



Dipnoi and Amphibia, and as already stated, it has a respiratory 

 function in some cases. 



An air-bladder is wanting in Cyclostomes and Elasmobranchs. 

 _ Attention has already been directed to the relations which often 

 exist between the air-bladder and the auditory organ (see p. 226). 



Fig. 225. — Internal Sukface of the 



AlB-BLADDEE OF LePIDOSTEUS, 

 SHOWING THE TkABECUL^. 



B, fibrous longitudinal band. 



2. The Lungs. 



The lungs arise at the hinder border of the branchial region of 

 the pharynx, which here becomes divided by a longitudinal hori- 

 zontal fold into a dorsal and a ventral portion, the latter of which 

 gives rise to a blind sac, opening anteriorly by a wide aperture into 

 the former and composed of endoderm surrounded by mesoderm (Fig. 

 226). A longitudinal vertical furrow is then formed, dividing this 

 primitive lung-sac into right and left halves : the narrower proximal 

 portions of these represent the primitive hronchi, which communicate 

 with the pharynx by a single tube, the primitive windpipe or 

 trachea. The proximal end of the latter usually becomes differen- 

 tiated to form a larynx, or organ of voice, which opens into 

 the pharynx on its ventral side by means of a slit-like aperture, the 

 glottis. The lungs are therefore phylogenetically older organs than 



^ In the Gymnodonts (e. g. , Diodon, Tetrodon), the whole oesophagus is capable 

 of great distension. 



