AIR-BLADDER OF FISHES. 493 



to the lung is completed in the Lepidosiren, in which the 

 cellular subdivision and multiplication of the vascular surface are 

 combined with a complete bilateral partition of the bladder into 

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

 pulmonary artery, and with the communication of the ductus 

 pneumaticus, as in the Polypterus, with the ventral surface of the 

 oesophagus. 



At the first introduction into the Animal Kingdom of a true 

 lung, or air-breathing 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 Dermopteri, Holocephali, and Plagiostomi ; 

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

 order of Anacantliini, and not in the other family (PleuronectidcB) ; 

 here Ave can associate its absence with the peculiar flattened 

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

 account for the absence of the air-bladder in the Angler (Lophius), 

 which habitually keeps the sea-bottom : but the mechanical expla- 

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

 is not so obvious in regard to the Acanthopterous genera Percis, 

 Percophis, Eleginus, Auxis, Tracliypterus, and Gymnetrus. A 

 large and often complex air-bladder exists in most of the Siluroid 

 fishes ; but the genera Loricaria, Rhinelepis, and Hypostoma are 

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

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

 Polynemus 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, 

 wdiich is shown by its place of communication 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 place of communication with the alimentary tract. In the 

 Herring the ductus pneumaticus 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 



