298 



FISHES 



production. In the different groups of Fishes in which it is pre- 

 sent the air-bladder frequently undergoes remarkable structural 

 modifications and becomes adapted for various distinct functions. 

 In the Cyclostomata there is no trace of an air-bladder, and, 

 unless represented in certain Sharks (e.g. Mustelus, Galeus, and 

 Acanthias\ l by a small caecum embedded in the dorsal wall of 

 the oesophagus and communicating with its cavity, it is also 

 absent in all Elasmobranchs. In the Crosso- 

 pterygii (e.g. Polypterus)? the air-bladder is 

 double, but while the right sac is long and 

 somewhat tubular, the left is much smaller 

 and oval in shape (Fig. 174). Near their 

 anterior extremities the two sacs fuse into a 

 single unpaired chamber, beyond which they 

 again project in the form of two short caeca. 

 The median chamber opens into the oesophagus 

 on the ventral side by an orifice (c/V) bounded 

 by prominent lips and furnished with a mus- 

 cular sphincter. The organ is devoid of internal 

 sacculations. In the Chondrostei (e.g. Acipenser) 

 the air-bladder is oval in shape, with a smooth, 

 non-sacculated, inner surface, and a lining of 

 ciliated epithelium, and it communicates with 

 the oesophagus by means of a relatively wide, 

 dorsally placed, funnel-like orifice. 



In the Lepidosteidae the single air-bladder 

 extends the whole length of the abdominal 

 cavity, and, as in Polypterus, communicates with 

 the exterior through a larynx -like vestibule 



FIG. 174. Air-blad- 



der of Polypterus. provided with a glottis/ which, however, opens 

 dorsall F into the oesophagus (Fig. 175). A 

 strong fibrous band runs along the median line 

 of the inner surface of its dorsal wall, from which extends 

 ventrally on each side a series of transverse fibro- muscular 

 ridges, forming the boundaries of a double row of regularly 

 arranged alveoli (Fig. 176). The bottom of each alveolus 



1 Miklucho-Maclay, Jen. Zeitsch. iii. 1867, p. 448. 



2 Wiedersheim, Lehrb. d. vergl. Anat. d. Wirbelth. ed. 2, Jena 1886, p. 616. 



3 The glottis is furnished with a structure analogous to the epiglottis-like plate 

 of Protopterus (Wiedersheim, op. cit. p. 616). 



