250 PISCES. 



chial cavity, or extend thence to beneath the lateral muscles ; they 

 receive branches from the branchial arteries, and their veins enter 

 the aorta. 



With the above organs we may also include the hollow arbores- 

 cent tufts of accessory branchiae which lie, in Heterobranchus anguil- 

 laris, behind the true gills, as also the labyrinthine accessory gills of 

 Anabas, Osphromenus, Ophiocephalus, and others ; a part of the 

 upper pharyngeal Maxilla is here divided into a greater or less num- 

 ber of leaflets, from between which cells arise, wherein water can be 

 retained for a long time. These fishes form a peculiar family, the 

 Pharyngii Labyrinthiformes, and are able by this structure to live a 

 long while on dryland; the arteries of these organs proceed from 

 those of the gills ; the veins enter, after the analogy of the branchial 

 veins, into the aorta. 



THE SWIMMING-BLADDER. 



FREQUENTLY as it has been compared with the lungs of the higher 

 Vertebrata, and certainly, from its mode of development, position, and 

 internal structure, reminding us exceedingly of these organs in the 

 Amphibia, still the disposition of its vessels forbids our regarding 

 the Swimming-bladder as an instrument of respiration, and thus we 

 are still in doubt as to its precise functions ; it occurs only in the 

 Bony Fishes, but not in all the genera, and among the Cartilaginous 

 in the Sturgeon alone, which forms the transition-link to the Osseous 

 Fishes. The swimming-bladder must, however, exist in connexion 

 with definite modes of life in several Fishes, since it is frequently 

 absent in different species of a genus, or in nearly allied genera, c. g., 

 Scomber scombrus, Polynemus paradiseus, and the genera Pleuro- 

 nectes and Lophius. 



In ordinary cases the swimming-bladder is situated beneath the 

 spinal column, to which it is firmly attached by cellular tissue, and, 

 covered by the kidneys, overlaps the intestinal canal. It consists of 

 two coats ; of an external, which is very tough, fibrous, and glisten- 

 ing, and an internal or soft vascular mucous membrane. It is in- 

 vested upon its lower surface, or that facing the viscera, by perito- 

 neum. The swimming-bladder in some instances, as in Esox, Ga- 

 dus, ttolocentrus, Cepola, is of very great length, extending through 

 the whole body ; in others, as in the Eels, it is very short. Its lon- 

 gitudinal dimensions usually predominate over the transverse, the re- 

 verse, as in Silurus and Orthagoriscus oblongus, being of rare occur- 



