STRUCTURE AND SYSTEMATICS 67 



Varying degrees of adipose tissue investment have been found around the swimbladders of adults 

 of the hatchet fish, Polyipnus laternatus. This is shown in Text-fig. 35. 



The top drawing represents a swimbladder from a 32-5-mm. fish and the sac has a relatively thin 

 layer of fat around it. The middle and lower swimbladders came from 26-5 and 33-5-mm. fishes. In 

 these the swimbladder is buried in a mass of fat, the outline of which has some congruence to that of 

 the sac. As in Cyclothone, the fat was surrounded by darkly pigmented peritoneum. 



Text-fig. 34. Cross-sections through the fat-invested swimbladder of Cyclothone livida. On the left is shown the reticular 

 fat-charged tissue surrounding the regressed swimbladder, which is shown enlarged on the right and has a diameter of about 

 300//. crm, remains of capillaries of rete mirabile ; pp, pigmented peritoneum; rft, reticular fatty-tissue; rsb, regressed swim- 

 bladder ; rgg, regressed gas-gland cells ; sm, submucosa ; te, tunica externa. 



Text-fig. 35. Different degrees of fat-investment of the swimbladder of Polyipnus laternatus. The top swimbladder ( x 7-5), 

 showing the least degree of investment, is from a fish of standard length 32-5 mm. The middle one ( x 8) comes from a 

 26-5-mm. fish, while the bottom one ( x 6) is from a 33-5-1™™. specimen, ft, fatty investment of swimbladder ; gg, gas-gland; 

 pp, pigmented peritoneum; rm, rete mirabile; sbw, swimbladder wall. 



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