THE SWIMBLADDER AS A HYDROSTATIC ORGAN 81 



Text-figs. 15-23). Its arterial supply usually comes from a branch or branches of the retial artery, 

 while the venous return is through a vessel draining into the cardinal vein. 



Order Miripinnati 



In these teleosts the swimbladder is only functional during the larval phase (Bertelsen and Marshall, 

 1956). As in certain percomorph fishes, the anterior, thick-walled secretory part is sharply distinct 

 from the posterior thin-walled section, but there is no intervening diaphragm. Although it has not 

 been closely examined, there can be little doubt that the thin-walled part is concerned with gas- 

 resorption. 



Order Berycomorphi, Suborder Anoplogastroidea 



Both the Melamphaidae and Stephanoberyx have a typical ' oval ' which is set on the roof of the swim- 

 bladder near its posterior end. A drawing of the almost closed oval of a Melamphaes megalops is 

 shown in Text-fig. 27. In the specimen of M. unicornis (Text-fig. 28) examined, the oval, from which 

 a large vessel drains into the cardinal vein, was completely closed. The oval of the Stephanoberyx 

 monae was in much the same condition as that of Melamphaes megalops. 



Order Percomorphi 



Chiasmodontidae. Chiasmodon niger. In this species a diaphragm divides the anterior gas-secreting 

 chamber from the posterior resorbent chamber. Such a partition is found in the Solenichthyes, 

 Thoracostei, certain Percomorphi and Scleroparei (Fange, 1953 and personal observation) and is 

 probably quite common in zeomorph fishes (a diaphragm is found in Zeus faber). As Fange (1945, 

 1953) has shown, the movements of this diaphragm are controlled by the antagonistic action of the 

 secretory and resorbent parts. During gas-production the muscle fibres of the secretory part relax, 

 while those of the resorbent mucosa are contracted. The antagonism reverses during gas-resorption. 

 Considering first the latter, the contraction of the muscles in the secretory chamber pulls the dia- 

 phragm to the forward end of the swimbladder, while the relaxation of the resorbent layer ensures that 

 the capillary layer is expanded and fully exposed to the gases. During secretion the diaphragm moves 

 to the posterior end of the sac and now the gland is fully expanded. 



The swimbladders of the three Chiasmodon examined each show different phases of this process 

 (Text-fig. 29). In the 104-mm. fish the gland is bunched up and its chamber occupies no more than 

 one-fifth of the total length of the swimbladder. This fraction is rather more than a half in the 49-mm. 

 fish, while in the third fish it is a quarter. Having regard to the previous paragraph, it is evident the 

 swimbladder of the first and third fishes show the disposition of the tissues during resorption and 

 secretion respectively. 



BATHYPELAGIC FISHES WITHOUT A SWIMBLADDER 



Summarizing the earlier descriptive section, the following groups either lack a swimbladder, or it is 

 regressed in the adult phase: 



Order Isospondyli. Suborder Stomiatoidea : Gonostomatidae, Cyclothone spp., Gonostoma elongatnm, 

 G. bathyphilum; Astronesthidae, (in Diplolychnus mononema, Borostomias antarcticus, Astronesthes 

 gemmifer, the swimbladder seems to regress during adult life) ; Stomiatidae ; Melanostomiatidae ; 

 Chauliodontidae ; Idiacanthidae ; Malacosteidae. 

 Suborder Salmonoidea: Bathylagidae. 



Suborder Clupeoidea: Alepocephalidae (many of these fishes appear to be benthic in habit); 

 Searsidae. 



