STRUCTURE AND SYSTEMATICS 65 



If the sac is compressed to half its buoyant capacity, the volumes of the gaseous and gelatinous 

 phases will differ by no more than a factor of five. This assumes that the volume of the submucosa 

 is unchanged. But according to Le Chatelier's theorem, colloidal gels may be expected to take up 

 more water if subjected to an increase in pressure (Johnson, Eyring and Polissar, 1954). At all events, 

 a study of this aspect would be of interest. 



Some indication of the flow-patterns in the submucosa of a compressed swimbladder was found in Vin- 

 ciguerria. Sections were cut through two swimbladders, one of which was well expanded, the other com- 

 pletely relaxed. These are shown in Text-fig. 32 and PI. I, fig. 1 . The lines of flow are indicated by the 

 undulations of the fibres in the submucosa. The great change in the shape of the gas-gland cells can 

 also be seen. While looking at these sections I was reminded of the experiments described by Le Gros 

 Clark (1945). An inflated rubber balloon, the surface of which was lightly oiled, was coated with a 

 plastic material, such as gelatin or collodion. As the balloon was deflated, small elevations appeared on 

 the surface of the plastic, each of which sent out a triradiate pattern of anticlines as the contraction pro- 

 ceeded. Such elevations were found in the submucosa of the relaxed swimbladder (see PI. I, fig. 1) and 

 under each were arches of fibres, looking not unlike the strata forming an anticline in the earth's crust. 



Turning finally to the gas-proofing properties of the swimbladder wall, Fange (1953) has clearly 

 shown that it is the ' secretory mucosa ' which is impermeable to gases. The secretory mucosa is the 

 inner epithelium of the secretory part of the sac. During gas-production this tissue is in a relaxed 

 state and covers the inner surface of the bladder (the resorbent part being contracted). It is hardly 

 surprising that this should be so, for the other parts of the swimbladder carry many blood-vessels 

 through which gases would be lost. 



Evidently the inner epithelium is the 'inner tube', the connective-tissues being merely the 'tyre'. 



Fat-invested swimbladders 

 The presence of a fat-invested swimbladder in a deep-sea fish was first discovered by Ray (1950). In 

 her study of the peripheral nervous system of the myctophid Lampanyctus leucopsaras, she figures 

 (pi. 12, fig. 30) a transverse section through the trunk at the level of the swimbladder. Like a normal 

 gas-filled organ, it lies immediately below the kidneys and is surrounded by peritoneum, which is 

 darkly pigmented, as is usual in deep-sea fishes. The organ is filled with reticular connective tissue 

 having all the appearance of a system of fat-storing cells. Barham (1957) has given these further 

 details: 'In the adult state the bladder is largely filled by fatty connective tissue. A well developed 

 gas-gland is present and almost fills the reduced lumen, but in some specimens a gas bubble may 

 be present.' He also records that another myctophid, Diaphus theta, has a similar type of swimbladder. 

 As already stated in the descriptive section, fat-invested swimbladders are found in a number of 

 bathypelagic fishes. These are: 



Suborder Stomiatoidea: Family Gonostomatidae, Cyclothone spp., Gonostoma elongation; Family 

 Sternoptychidae, Polyipnus laternatus; Family Astronesthidae, Borostomias antarcticus; Family 

 Stomiatidae, Stomias colubrinus, S. affinis. Order Berycomorphi: Family Anoplogastridae, Anoplo- 

 gaster longidens. 



Kotthaus (1952) has also found a well-formed fat-bearing swimbladder in another deep-sea beryco- 

 .morph, Hoplostethus islandicus (family Hoplostethidae). 



It will thus be seen that fat-invested swimbladders are found in deep-water fishes belonging to 

 seven different families and three orders. 



As already mentioned, the post-larval stages of Cyclothone have a small gas-filled swimbladder. It 

 is after metamorphosis that the swimbladder regresses and becomes invested with adipose tissue. In 



