4 66 



ELEMENTAR Y ANA TOMY. 



[LESS. 



It may abort altogether in the adult (the swim-bladder thus 

 becoming quite separate from the alimentary tube), as in the 

 Cod, Perch, and very many others. 



In shape the swimming-bladder may bifurcate anteriorly 

 as in Ludo-perca, or posteriorly as in Acanthurusj or it 

 may be constricted towards its middle, as in the Carp ; 

 or it may send out two blind processes' on each side, 

 as in Corvina; or a multitude on each side, as in Johnius. 

 Finally, the swim-bladder may give out processes from 

 each side, each process bifurcating dorsally and ventrally, 

 and each dorsal and ventral bifurca- 

 tion again dividing and anastomosing 

 with processes of the opposite side, 

 the whole complex structure being 

 invested by folds of the peritoneum. 

 This maximum of complexity is ex- 

 hibited by Callichthys. 



It may be connected with the in- 

 ternal ear, as in the Carp (Cyprinus) 

 and Loach (Cobitis), by the interven- 

 tion of small special ossicles analo- 

 gous to, but not homologous with, the 

 auditory ossicles of man. In the 

 Loach the bladder seems chiefly 

 related to some auditory function, as 

 it only extends to the third vertebra. 



The cavity of the swim-bladder 

 may be simple, as is usually the case ; 

 it may be divided by septa, as in 

 Bagrus ; it may, finally, be so sub- 

 divided into cells as to be like the lung 

 of a Reptile, as is the case in Amia. 

 Retia mirabilia may exist in the swim-bladder, as in the Cod. 

 Every representative of a lung may be absent, as in the 

 Sharks, Rays, Pleuronectidce, Marsipobranchii, and Ajnplii- 

 oxus, in all of which there is no swim-bladder. 



The mechanism of aerial, pulmonic respiration, which exists 

 in man, exists also in all Mammals the diaphragm being 

 their main agent in filling and emptying the lungs. 



The lungs, however, may be filled and emptied by alternate 

 movements of the sternum towards and away from the verte- 

 bral column. This is the case in Birds (Fig. 76), where such 

 movements are aided by the synovial articulations between 

 the sternal and vertebral ribs. 



FIG. 394. AIR-BLADDER OF 

 THE TELEOSTEAN FISH, 

 Johnius lobatus. 



