AIll-BLADDER OF FISHES. 493 



to the king is comjjleted in tlie Lepidosircn, in which the 

 celluhir subdivision and niuUiplicatiijn oi' the vascukir surface are 

 comhined witli a complete hilatcral partition of the bhidder into 

 two elongated sacs, with a supjdy of venous blood from a true 

 pidmonary artery, and with the connnunication of the ductus 

 l)neumaticus, as in the Polypterus, with the ventral surface of the 

 tt'sophagus. 



At the first introduction into the Animal Kingdom of a true 

 lung, or air-ljreathing organ communicating with the pharynx or 

 oesophagus, much variety of form and structure, much inconstancy 

 even as to existence, might be expected, especially in that class in 

 which the normal function of the new organ could be so seldom in 

 any degree exercised, and in which, therefore, different accessory 

 or subordinate (offices predominate in such rudimental represen- 

 tative of the pulmonary organ. There is no swim-bladder, for 

 example, in the orders Denno]/teri, Iloloo-pliali, and Plagiostomi ; 

 it is present in one of the families ( Gudidce) of the thoracic sub- 

 order of Anacantldni, and not in the other family {Plcuronectldoi) ; 

 here we can associate its absence with the peculiar flattened 

 form and grovelling habits of the species. In lilce manner we may 

 account for the absence of the air-bladder in the A)igler (^Lopluun), 

 which lialntually keeps the sea-bottom : Ijut the mechanical expla- 

 nation of the absence or rudimental condition of the swim-bladder 

 is not S(j obvious in regard to the Acanthopteroiis genera Fercis, 

 Percophis, Elegmus, Auxis, TracliyiiteruH, and GymvetruH. A 

 large and often comjilex aii'-ldadder exists in most of the Siluroid 

 fishes ; but the genera Loricuria, Blihtelepis, and Hi/postonui. are 

 exeejitions in that family, having no air-bladder. What is more 

 inexplicable is, that while some spiecies of the same genus, 

 Pohjnemus and Scomber for example, have a large swim-bladder, 

 others want it, or have it of extremely small size. 



The variation in respect to the presence or absence of an air- 

 duct {ductus pueumaticus) is expressed in the characters of the 

 orders in the Classification of Fishes, pp. 10 — 12. The duct, 

 wliich is shown by its place of communicaticm with the beginning 

 of the oesophagus, and by the rudimental larynx, in Polypterus 

 and Lepidosiren, fig. 316, e, to be the homologue of the trachea 

 of air-breathing Vertebrates, is a simple and delicate membranous 

 tube ; but it presents considerable variation in its length, diameter, 

 and j)lace of communication with the alimentary tract. In the 

 Herring the ductus pncumaticus is produced from the posterior 

 attenuated end of the cardiac division of the stomach, fig. 281, i, 

 and opens into the fusiform air-bladder at the junction of the 



