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 cesophagus 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- 
Fic. 255.—Air bladder of Megalops cyprinoides, after de Beaufort. uw, anus; }, air 
bladder; d, pneumatic dust leading from the cesophagus; /, ligament; p, anterior part of 
bladder extending to skull. 
ately below the vertebre 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 
E 
5 d 
sat SB Ze ee 
oe oe oe 
Fic. 256.—Swim-bladders of physostomous fishes; A, pickerel (Esox); B, carp (Cypri- 
nus); and C, eel (Anguilla) after Tracy. 6, swim-bladder; d, duct; g, red gland; oe, 
cesophagus. 
life by the pneumatic duct. This usually empties into the cesophagus, 
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 
