62 DISCOVERY REPORTS 



peritoneal coat. This is formed of strong close-set fibres that tend to run along the major axis of 



the swimbladder. 



Astronesthes niger 



The swimbladder described on p. 23 was examined under a high-power binocular, the tissue layers 

 of the walls being teased apart. The sac is completely enveloped by the peritoneal coat, which is 

 constructed of close-set fibres that are set along the major axis. The underlying tunica externa is 

 formed of circularly disposed fibres. Next follows the thick submucosa with its loose network of 

 fibres that run in all directions. 



Family Myctophidae, Myctophum punctatum 



A swimbladder measuring about 12 mm. in length and 5 mm. in depth was sectioned. 



The peritoneum extends round the floor and lateral walls of the sac. It is formed of lamellar 

 collagen fibres (about 5-10// in width) which run along the major axis of the sac. This layer carries 

 melanophores. The underlying tunica externa is barely detectable, but is comprised of fibres that run 

 round the swimbladder. As is usual, the submucosa forms most of the wall thickness, this being 

 about 300/* under the gas-gland and about 50// in the roof of the sac. Remains of the ground substance 

 could be seen adhering to the loose fibrillar network. Here and there the cells of the ground substance 

 could be detected. Finally, there is a filmy coat of pavement epithelium lining the inner surface 

 of the sac. 



Diaphus rafinesquei 



Certain details of the structure of the swimbladder wall have been described by Rauther (1922). As 

 in Myctophum punctatum, the fibrous peritoneum (' fibrose hiille ') does not completely invest the sac 

 but extends rather more than half way up the lateral walls. Between the peritoneum and what 

 Rauther drew as a thick circular layer of fibres is a padding of loose, reticular connective tissue. The 

 circular layer must represent the tunica externa and the submucosa. Microscopic examination of the 

 swimbladder of the fish from Discovery station 3484 (see pp. 30-32) revealed other structural details. 



At least at the forepart of the organ, the peritoneal coat extends over the roof of the sac. It is 

 formed of fine and close-set collagen fibres together with a number of elastic fibres. The tunica 

 externa consists of relatively strong fibres running round the sac, while the more delicate fibres of the 

 submucosa form a loose network. As far as could be judged the lateral walls are about 50^ in thick- 

 ness, the submucosa accounting for about nine-tenths of this dimension. 



To summarize, the wall of the swimbladder usually has this structure : a voluminous loosely- woven 

 reticulum of collagen fibres in a gelatinous ground-substance separates the thin, but tough outer 

 fibrous layer from the inner, filmy epithelium lining the sac walls. Clearly this tissue-complex must 

 have adequate mechanical, as well as gas-proofing qualities. 



In the steady state any loss of gas by diffusion out of the sac can be countered by the activity of the 

 gas-gland. Presumably the walls will be under tension without undue strain. It would also seem 

 likely that the fibres of the submucosa will lie close together within the gelatinous matrix, so forming 

 a densely matted tissue, not the loose reticulum seen in prepared sections. 



Concerning the gas-proofing qualities, a fish living at a depth of 500 m. will most probably have 

 80 per cent of oxygen in its swimbladder gases, so the swimbladder wall must contain a pressure of 

 40 atmospheres of oxygen against the tensions of this gas in the body fluids. (About 0-2 atmosphere 

 for arterial blood.) Evidently, the swimbladder wall must be remarkably impermeable to oxygen. 



