603 



the following characteristic differentiation. The wall of the swim-bladder 

 itself is lined with a single layer of columnar epithelium which is 

 thrown up in simple folds as shown in cross-section (Fig. 4a). In each 



oes 



Fig. 4. Diagram of the swim-bladder in the eel. A anterior end of the oeso- 

 phagus, oes oesophagus, pnd pneumatic duct, sb swim-bladder. 



of these folds run one or more small blood vessels. The pneumatic 

 duct, on the other hand, is lined with flat epithelium, immediately 

 under which lies a "rete mirabile" (Fig. 4b). 



Fig. 4 a. 



Fig. 4 a. Section of wall of the eel 

 swim-bladder (JACOBS, '98). sbl cavity of 

 swim-bladder, epi epithelium lining the 

 swim-bladder cavity, cp small blood vessel. 



Fig. 4 b. Section of wall of pneu- 

 matic duct in the eel (Jacobs, '98). bv 

 blood vessel, cp capillary, epi epithelium 

 lining cavity of the pneumatic duct, pnl 

 cavity of pneumatic duct. 



Fig. 4 b. 



A more specialized type of swim-bladder is found in the toad-fish 

 (Opsanus). The organ of this type (Fig. 5) has lost its connection with 

 the oesophagus, and consists of a closed vesicle, divided into an ante- 

 rior and a posterior chamber by means of a transverse partition. Near 

 the center of this partition is a small round opening by means of 

 which the two chambers are put into communication. The partition 

 is formed by a fold which includes only the epithelial, inner and middle 

 layers. In the wall of the posterior chamber the inner layer is very 



