362 



COMPARATIVE ANATOMY 



more or less sac-like in form, is only exceptionally paired, and 



usually extends along the whole length of the body-cavity ; its 



walls are composed of connective, 

 elastic, and muscular tissue. 1 In 

 some Teleostei the swim-bladder 

 is transversely constricted so as 

 to form several successive divi- 

 sions ; in other cases it may give 

 rise to a more or less numerous 

 series of caecal processes. Its 

 internal surface may be either 

 smooth or spongy (Fig. 267) ow- 

 ing to the formation of a mesh- 

 work of trabeculse, the structure 



FIG. 267. INTERNAL SURFACE OF of which resembles that of the 

 THE AIR-BLADDER OF LEPIDOSTEUS, i un g s o f Dipnoi and Amphibia, 



and, as already stated, it has a 

 respiratory function in some 

 cases. 

 Attention has already been directed to the relations which 



may exist between the swim -bladder and the auditory organ 



(cf. p. 297). 



SHOWING THE TRABECULJE. 



B, fibrous longitudinal band. 



2. THE LUNGS. 



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

 the pharynx. Their phylogenetic history is not clear, and the 

 view that they arose primarily from visceral clefts does not seem 

 to be a very probable one. 



On the first appearance of the lung-rudiments, the pharynx 

 becomes laterally compressed immediately above the fifth or sixth 

 arterial arch, and divided by a longitudinal horizontal 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 lined by endoderm (Fig. 268). 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 'bronchi, which communicate with the pharynx by 

 a short tube, the primitive windpipe or trachea. The proximal 

 end of the latter subsequently becomes differentiated 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 the bronchi, 



1 In Dactylopterus volitans, the two halves of the swim- bladder extend 

 beyond the general ccelome into special cavities, and the organ is covered by 

 large bony plates. In the Gymnodonts (e.g. Diodon, Tetrodon), the whole 

 oesophagus is capable of great distension. 



