. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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 recon
. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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
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