274 LECTURE XI. 



and not in the other family (Pleuronectidat) ; here we 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 (Lop/iius), which habitually keeps 

 the sea-bottom : but the mechanical explanation of the absence 

 or rudimental condition of the swim-bladder is not so obvious in 

 regard to the Acanthopterous genera Percis, Percophis, Eleginiis, 

 Auxis, Trachypterus, 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 pneumaticus) is expressed in the characters of the orders 

 in the Classification of Fishes, pp. 48 — 50. The duct, which is shown 

 by its place of communication Avith the beginning of the oesophagus, 

 and by the rudimental larynx, in Polypterus and Lepidosiren, to be the 

 homologue of the trachea of air-breathing Vertebrates, is a simple and 

 delicate membranous tube ; but it pi-esents considerable variation in 

 its length, diameter, and place of communication with the ali- 

 mentary tract. In the Herring the ductus pneumaticus is j)roduced 

 from the posterior attenuated end of the cardiac division of the 

 stomach*, and opens into the fusiform air-bladder at the junction 

 of the middle and posterior thirds of that organ. The long, narrow, 

 and flexuous ductus pneumaticus is continued from the fore-part of the 

 posterior division of the air-bladder in the Cyprinoids, and opens into 

 the dorsal part of the oesophagus {fig. 58. sii) : the short, straight, 

 and wide ductus pneumaticus, in the Lepidosteus, opens also into the 

 dorsal part of the oesophagus, the orifice being served by a sphincter : 

 in the Erythrinus the air-duct communicates with the side of the 

 oesophagus ; in the Polypterus, with the under or ventral part of the 

 beginning of the flesophagus.f 



The principal seat of the vascular ramifications in the air-bladder, 

 like that in a true lung, is the mucous lining membrane ; but 

 the modes of ramification in the primitive piscine form of the air- 

 breathing organ are as variable as any of its other properties. The 

 arteries of the air-bladder are derived sometimes directly from the 



* cxiv. ii. pi. viii. fig. 1. 



•]• These variations show how futile is the objection drawn from the dorsal com- 

 munication of the swim-bladder in Lepidosteus to the determination of the Lepido- 

 siren to the class of fishes, and of the homology of its lungs with the swim-bladder 

 in that class. 



