AIE-BLADDER OF VERTEBEATA. 567 



organ^ and the air contained in tlie organ itself. This must happen 

 whenever the blood brought to the organ is less rich in oxygen than 

 the air in the organ. The changes in the vascular system do not 

 occur till this has happened. 



The pneumatic organs of the enteric tube are divided, therefore, 

 into two series which are functionally very different, though morpho- 

 logically homologous, and each series undergoes a large number of 

 differentiations. 



a) Air-Bladder. 

 § 424. 



This organ is not found in Amphioxus, nor in the Cyclostomata. 

 In some Sharks (Galeus, Mustelus, Acanthias) there is a diverticulum 

 of the dorsal wall which opens into the pharynx, and which may be 

 regarded as the rudiment of an air-bladder. All the G-anoidei, and 

 very many of the Teleostei, have air-bladders. If we examine the 

 arrangements which obtain in the Gano'idei more closely, we find 

 that there is a single or paired sac, which is connected with the 

 pharynx by an air- duct of varying length. This opens on to the 

 upper wall of the fore-gut, and, generally, at the same point as that 

 at which we find the short ctecal sac in the Selachii. In Acipenser 

 the opening is placed very far back, and the air-bladder is connected 

 with the stomach. In Polypterus we find a paired air-bladder 

 (Fig. 321, A), which opens on to the ventral wall of the oesophagus ; 

 in Lepidosteus it is placed on the dorsal surface, and, though single 

 externally, is divided into two longitudinal halves by the trabecule! 

 which traverse it, and each of these halves is again divided into 

 smaller cellular cavities by a number of processes and bars; the 

 inner surface is thereby greatly increased in size. In Amia, also, 

 the cellular air-bladder is divided by a fold, and is continued 

 anteriorly into two short cornua. In the three last-mentioned 

 Ganoids the bladder opens into the enteron by a short and some- 

 what narrow air-duct which leads to a longitudinal cleft. Even in 

 the Ganoidei, therefore, there are great variations in the characters 

 of the air-bladder, and their significance must be estimated in rela- 

 tion to the fact that this division now consists of but few living 

 forms. It is very significant that in the various stages of the 

 air-bladder in the Ganoidei all the essential arrangements are 

 recognisable which are presented by this oi:gan, either under the 

 form of the air-bladder of the Teleostei, or the lung of the higher 

 Vertebrata. 



In one division of the Teleostei the air-duct is persistent (Phy- 

 sostomi); in the rest it is developed for a time only, for it disappears 

 again after the development of the air-bladder (Physoklisti) ; in 

 many, finally, the air-bladder ceases to be developed. This last 

 character is, moreover, very variable, even within the limits of 



