64 DISCOVERY REPORTS 



Perhaps these differences between perch, cod and bathypelagic teleosts have a functional rather 

 than a mechanical basis. A cod being hauled to the surface in a trawl is subjected to pressure changes 

 beyond those it normally experiences. This would not apply to those bathypelagic fishes which 

 migrate upward each day to the surface-waters. However, discussion of this will best be left to the 

 section dealing with gas resorption (pp. 78-81). 



Text-fig. 32. Semidiagrammatic reconstructions of the swimbladder of Vinciguerria in (a) an expanded, and (b) a com- 

 pressed state. Note the changes in shape of the gas-gland cells (shown black) and the retia mirabilia (rm). The undulating 

 patterns in the submucosa of the compressed swimbladder may be better seen in the microphotograph in PI. I, fig. 1. Below 

 are shown cross-sections of the capillaries of the retia (vc, venous capillaries, others arterial). In those belonging to (a) both 

 sets of capillaries are expanded, as during gas secretion: in (b) the arterial capillaries are occluded, as during gas resorption. 



During the migration of a fish towards the surface, the physical properties of the swimbladder wall 

 must be adequate to withstand any stresses that might arise. But the walls will presumably be under 

 tension without undue strain. It would also seem likely, as we have said, that the fibres of the sub- 

 mucosa will lie close together within their gelatinous matrix. When the fish dives to its daytime level, 

 certain evidence suggests that the swimbladder cannot be kept at the required volume in the face of 

 pressure gradient (p. 91). As the swimbladder is compressed the volume of the fish will decrease and 

 it would seem desirable that the tissues of the walls should be freely adjustable as the sac decreases 

 in size. If, for instance, the connective tissues were ' rigid ', with little play, kinks would appear in the 

 wall which might eventually lead to one part of the delicate gas-gland being forced against another 

 (see Text-fig. 32). However, the presence of a voluminous, semi-fluid submucosa will allow the 

 tissues to relax in a uniform manner and so the swimbladder may keep its shape. In a large Diaphus 

 rafinesquei, the swimbladder submucosa is about 150// in thickness, and in a sac with major and 

 minor axes of 12-0 and 7-0 mm., the volume of this tissue will be rather more than one-tenth of that 

 of the gas. 



