Characteristics of the Vertehrata. Ixxv 



is functionally all but exclusively hydrostatic, may be considered to be 

 established by a comparison of those lung-like air-sacs with the air- 

 bladder of the Ganoid Polypterus, which is somewhat similai-ly bifid, 

 and opens similarly into the pharynx by an air-duct entering it on its 

 ventral surface. From the air-bladder of the Polypterus to that of the 

 Lepidosteus, which however opens into the pharynx from its dorsal side, 

 and which, though divided internally into two longitudinal compartments, 

 is yet externally a single sac, the transition is not abrupt ; nor that from 

 such air-bladders as those of the Lepidosteus and the Physostomatous 

 Teleostei, to the ductless air-bladders of the Acanthopteri and other 

 bony Fish. 



The swimming bladder is developed as an outgrowth from the 

 oesophageal portion of the digestive tube^ and its ductus 2meuma- 

 ticus, when persistent, ordinarily communicates with this portion of 

 the tract. Both bladder and duct are absent in the Mars'qio- 

 b7-anchii, and, except as rudimentary structures in certain Sharks, 

 in all Elasi/iobrancliii also. The duct is aborted in the majority of 

 Teleostei (the Acanthopteri, Phari/ngognathi, Lopliohranchvi^ Plec- 

 to(jnathi\ but is present in the remainder of this order, nearly 

 corresponding to the Malacopterijgii of Cuvier, and hence called 

 Physostomi, as also in all Ganoidei and Dipnoi. With the presence 

 or absence of an air-duct to the air-bladder, the presence or ab- 

 sence of bone corpuscles appears to be nearly universally correlated. 

 The shapes which the air-bladder assumes are very various, es- 

 pecially when it is ductless. In many Acanthopteri it sends two 

 prolongations into relation with the caudal muscles, whilst in 

 many Pht/sostomi {Cyprinoideae, Siluroideae, Clupeidae), its ante- 

 rior prolongation is brought into relation with the auditory ap- 

 paratus. 



It is only in the Elasmobranchii that a secondary kidney takes 

 the place of the primordial Wolffian body, which remains as the 

 functional renal organ in other orders of Fishes. This diffei'ence 

 is illustrated not only by the difference of form and of compactness 

 of the renal organs in the Elasmobranchii, but also by the facts that 

 in the females of this order the oviducts open separately from the 

 ureters into the cloaca ; that in the males the vasa defcrentia are in 

 some species {Mustelus laevis) bestudded with what is probably the 

 remnants of Wolffian bodies for nearly their entire length; and 

 that the ureters are developed mainly along the internal, and not 



