SURVEY OF STRUCTURE 



49 



In the 50-mm. fish a transverse diaphragm was found in the posterior half of the swimbladder. 

 There is an aperture in the central part of the diaphragm, through which the anterior chamber con- 

 taining the retia and gas-gland, communicates with the posterior chamber. It is clear that the 

 structure of the swimbladder of Chiasmodon is essentially similar to that found in other physoclist 

 teleosts, in which a diaphragm separates an anterior gas-producing chamber from a posterior resorbent 



Text-fig. 29. Swimbladder of Chiasmodon niger (ventral view), seen in three different states of activity ; (a) with the resorbent 

 posterior chamber well expanded, (b) with the gas-secreting and resorbent chambers about equal in volume, and (c) with 

 the gas-secreting chamber well expanded, dm, diaphragm; gg, gas-gland; rm, rete mirabile. (a, x 6-9; B, x 19-5; c, x 18.) 



chamber. It is also evident that the relative volumes of these two chambers vary in the three specimens. 

 In the largest fish the diaphragm is close behind the gas-gland in the anterior third of the swim- 

 bladder, while it occupies a median position in the 49-mm. individual. These variations will be con- 

 sidered at greater length in the section concerned with gas resorption (p. 81). 



The swimbladder is absent in two other members of the Chiasmodontidae, Pseudoscopelus scriptus 

 and Dysalotus alcocki. 



