ORGANS OF RESPIRATION. 249 



sac or pouch, upon the internal walls of which strongly-developed 

 folds project, as in the Plagiostomi ; each sac opens externally by a 

 round aperture, and in the inward direction by a canal into the oesoph- 

 agus, or even into a special membranous tube or bronchus, situated 

 beneath the oesophagus, and opening anteriorly into the pharynx, 

 where it is closed by a membranous valve, and terminates blindly 

 posteriorly. 



The movement of the branchial arches, and also of the operculum 

 and branchiostegous membrane of the Osseous Fishes, is effected by 

 numerous muscles, which are absent in the Cartilaginous Fishes with 

 fixed gills. By their action the branchial arches are separated or 

 approximated, the branchiostegous membrane spread out, and the 

 operculum flapped to and fro so as to open or shut the external 

 branchial fissure. Smaller muscles move the double row of branchial 

 leaflets against each other. Similar muscular fasciculi are found in 

 the Cyclostomi, and serve to expand the branchial sacs. The water 

 taken in at and streaming through the mouth, is driven by the move- 

 ment of the branchial arches and hyoid bone between the gills, 

 where it bathes the leaflets with their superimposed plexuses of ves- 

 sels, and is again expelled through the external branchial fissures. 



In many Osseous Fishes, Branchial follicles, as they are termed, 

 that secrete a copious mucus, are found at the posterior commence- 

 ment of the branchial cavity. 



In addition to this mode of respiration by branchiae, we find that 

 in many Fishes this function is performed by pulmonary organs. 

 The Amphibioid Fishes, like Proteus among the true Amphibia, 

 possess a pair of truly-developed Lungs near to the gills. In Lepi- 

 dosiren annectens a partly single, partly double row of branchial 

 filiments project from the six branchial arches with the exception of 

 the second and third, and in the vicinity of the anterior extremity 

 we find the single branchial fissure. Besides these gills, however, 

 a double sacciform lung is present, each portion being divided into 

 several lobes ; it is situated behind the kidneys against the ribs, and 

 is internally cellular like the lung of a serpent ; anteriorly it opens 

 by a tolerably long, narrow, and membranous tube into the oesopha- 

 gus. Each lung receives a branch of the pulmonary artery which 

 arises from the branchial arteries. 



Among even the true Bony Fishes we meet with Accessory or pul- 

 monic organs of respiration, e. g., in Silurus fossilis of Bloch, and 

 among Fishes of the Eel kind in Amphipnous Cucia ; they consist 

 of vascular hollow sacs, which are either situated within the bran- 



