SWIM-BLADDER 361 



II. SWIM-BLADDER AND LUNGS. 



1. THE SWIM-BLADDER. 



As already mentioned (p. 351), the lungs and swim-bladder are 

 developed in a similar manner, and only differ from one another in 

 the fact that the former always arise from the ventral side of the 

 pharynx, and the latter usually on the dorsal side. The various 

 attempts which have been made to trace the phylogenetic 

 connection between swim-bladder and lungs have so far not been 

 completely successful. It has been supposed that the dorsal origin 

 of the former has come about by a process of rotation : on the 

 other hand, it may be that the two organs are not strictly 

 homologous, each having arisen independently from a similar 

 outgrowth at a different point in the alimentary tube. In this 

 case, the so-called swim-bladder of Polypterus would be directly 

 comparable to the lungs of Dipnoans and higher forms rather 

 than to the swim-bladder of other Fishes. 



The exact point of origin of the swim-bladder from the ali- 

 mentary canal varies, 1 and its duct (ducfus pncumaticns') may either 

 remain open throughout life, as in Ganoids and some Teleosts 

 {Physostomi}, or it may later become reduced to a solid fibrous 

 cord or even entirely obliterated, as in other Teleosts (Physodisti]. 

 In the latter case there is no communication between the swim- 

 bladder and the external air, and the contained gas must therefore 

 be given off from the walls of the swim-bladder itself: this has 

 been shown to be the case amongst Physostomi also. In certain 

 of the latter, vascular " red-bodies " are present ; in others and in 

 the Physoclisti, gland-tubules are present amongst the vascular net- 

 works or " retia mirabilia," and the number, relative size, and 

 position of these gas-glands varies in different forms. Diffusion 

 of gas (probably mainly oxygen) into the bladder takes place in 

 certain thin, oval regions lined by a flat epithelium and provided 

 with regulating dilator and sphincter muscles. 



A g'vim-bladder is wanting in Amphioxus, Cyclostomes, and 

 Elasmobranchs, and in other Fishes varies much in form and in its 

 relations to surrounding parts (alimentary canal, blood-vessels 

 skeleton). As a rule, it lies above the peritoneum on the dorsal 

 side of the body-cavity, between the vertebral column, aorta, and 

 kidneys on the one hand, and the alimentary canal on the other, 

 and is invested by the peritoneum on the ventral side only. It is 



martinicencis) or folds of the body-wall (Rana opisthodon). In Xeuopus, the 

 branched tentacles at the angles of the mouth are said to have a respiratory 

 function, and, like the "balancers" of Urodeles (cf. note on p. 271), may corre- 

 spond morphologically to the external gill of the first visceral arch. 



1 In Erythrinus it arises laterally, and in some Physostomi (e.g. Herring) 

 it opens further back, into the stomach. 



