SWIM BLADDER 



267 



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), 

 immediately 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 



Fig. 287. — Air bladder of Megalops cyprinoides, after de Beaufort, a, anus; b air 

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

 bladder extending to skull; s, stomach. 



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

 of the alimentary canal (fig. 233), and in the ganoids and one group 

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

 throughout life by the pneumatic duct (fig. 288). This usually emp- 

 ties 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 connexion with the digestive tract (physoclisti). 



I iG. 288. — Swim bladders of physostomous fishes; A, pickerel (Esox); B, carp 

 {Cyprinus); and C, eel {AnguiUa) after Tracy, b, swim bladder; d, duct; g, red gland; 

 oe, oesophagus. 



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

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

 (fig, 288). It is usually regular in outline, but diverticula of all kinds 

 are common, the form being most varied in the physoclistous species. 

 Internally the walls may be smooth and the cavity simple, or it may 

 be subdivided by septa (fig. 289), or, as in Amia and Lepidosteus 

 (fig. 290, B) , it may be alveolar, recalling the condition in the lungs 



