THE SWIMBLADDER AS A HYDROSTATIC ORGAN 75 



for the time it spends at its daytime level (say, 12 hr.). To make good this loss, the flying-fish must 

 secrete 1-5 ml. gas at a pressure of 1-5 atmospheres. The lantern fish must produce 0-0125 m ^ at 

 30 atmospheres or 0-25 ml. at 1-5 atmospheres. Thus, while the volume of the flying-fish swimbladder 

 is 120 times that of the lantern fish, the relative amounts of gas required to restore a gradual loss of 

 buoyancy differ by no more than a factor of 6. 



Table 5. Relative development of the gas-gland in some epipelagic and 



bathy pelagic fishes 



However, in estimating the figures in Table 5, it was found that a Cypsilunis of the above weight 

 has a gas-gland with a surface-area of 25 mm. 2 , while the figure for the lantern fish is about 20 mm. 2 

 Furthermore, the lantern fish has a gas-gland consisting of many layers of cells, whereas that of the 

 flying fish is probably single-layered. This would suggest that the lantern fish is readily able to make 

 up the loss of gas. But, unlike the flying-fish, the lantern fish undertakes extensive vertical migra- 

 tions. Towards sunset it will climb several hundred metres towards the surface-layers, and after 

 spending the hours of darkness near the surface, will then dive to its daytime depth. In considering 

 the relative development of the gas-gland, it is clear that vertical migrations must also be taken into 

 account. This will best be dealt with at a later stage (p. 89). Here we may turn to the fine structure 

 of the gas-glands. 



In shallow-water fishes there is considerable variability in the fine structure of the gas-glands 

 (Woodland, 191 1 a; Fiinge, 1953) and this is also true of deep-sea fishes. However, three main groups 

 may be recognized (see Text-fig. 37). 



(1) Gas-glands consisting mostly of giant cells. Capillaries partly intracellular. Vinciguerria and 

 Sternoptyx. In the first genus the cells of the expanded gas-gland measure from 100 to 150// in length. 

 Nusbaum-Hilarowicz (1920) gives the dimensions of the cells of Sternoptyx as 50-95//. In Vinci- 

 guerria the extent of the intracellular capillaries is considerably greater than that of the intercellular 

 elements, but the reverse seems to be true of Sternoptyx. 



(2) Gas-glands consisting mostly of medium-sized cells. Capillaries intercellular. Cyclothone, 

 20-50//; Maurolicus, 20-40//; Argyropelecus, 25-50//; Polyipnus, 25-50//. 



(3) Gas-glands with small cells. Capillaries intercellular. Myctophum punctatum, 15-20//; 

 Diaphus dofleini, 10-15//; Opisthoproctus soleatus, 15-25//. 



Giant cells are also found in the gas-glands of shallow-water fishes, such as Carapus, Perca and 



