298 FISHES CHAP. 
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),) 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 (eg. 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 (g/) bounded 
by prominent lps 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 
cihated 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 
Bie toate: the exterior through a larynx-like vestibule 
der of Polypterus. Provided with a glottis,’ which, however, opens 
iteasra | dorsally 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 
? Miklucho-Maclay, Jen. Zeitsch. iii. 1867, p. 448. 
* Wiedersheim, Lehrb. d. vergl. Anat. d. Wirbelth. ed. 2, Jena 1886, p. 616. 
* The glottis is furnished with a structure analogous to the epiglottis-like plate 
of Protopterus (Wiedersheim, op. cit. p. 616). 
