248 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



occurs only in teleostomes, and while found in most species (frequently 

 absent from bottom-feeding forms pleuronectids, etc.), it is lacking 

 here and there, even among species classed as physostomous (Lori- 

 caria, etc.). In the young of a few sharks (e.g., Scyllium) there is a 

 pouch on the dorsal side of the oesophagus which suggests the possible 

 origin of the organ. 



The swim bladder lies dorsal to the alimentary tract, outside of the 

 peritoneum (which frequently covers only its ventral surface) immedi- 



FIG. 255. Air bladder of Megalops eyprinoides, after de Beaufort, a, anus; b, air 

 bladder; d, pneumatic dust leading from the oesophagus; /, ligament; p, anterior part of 

 bladder extending to skull. 



ately below the vertebrae and excretory organs (mesonephroi) . In 

 some instances it extends the whole length of the body cavity and 

 (clupeids) may even send diverticula into the head. In other species 

 it may be much shorter. In development it arises as a diverticulum of 

 the alimentary canal (fig. 209), and in the ganoids and one group of 

 teleosts (physostomi) it is connected with the digestive tract throughout 



r 



oe 



oe 



oe 



FIG. 256. Swim-bladders of physostomous fishes; A, pickerel (Esox); B, carp (Cypri- 

 nus); and C, eel (Anguilla) after Tracy. b, swim-bladder; d, duct; g, red gland; oe, 

 oesophagus. 



life by the pneumatic duct. This usually empties into the oesophagus, 

 but it may connect with the stomach. In most teleosts, however, the 

 duct becomes closed at an early date and the bladder loses its connex- 

 ion with the digestive tract (physoclisti). 



The swim bladder is usually unpaired (paired in most ganoids) and 

 may be simple or divided into two (rarely three) connecting sacs (fig. 

 256). It is usually regular in outline, but diverticula of all kinds are 



