STRUCTURE AND SYSTEMATICS 61 



circular layer is a thick layer (the submucosa) of fibres forming a loosely-woven reticulum. As in 

 Stemoptyx, this fibre complex would appear to be developed within a gelatinous matrix. Last comes 

 the inner, filmy coat of pavement epithelium. 



Longitudinal sections were prepared of the specimen of Argyropelecus olfersii described on p. 20. 

 The tissue composition of the walls is much like that of A. hemigymnus. Above the gas-gland, the 

 roof has a thickness of 50-100//, most of this consisting of the submucosa. The floor of the sac under 

 the gas-gland is about three times as thick as the roof, but this swimbladder was in a very relaxed 

 state. When the swimbladder is taut and in the steady state, the walls must be a good deal thinner, 

 even remembering that the above measurements have been taken from tissues shrunken by pre- 

 servation and subsequent preparation. 



Family Gonostomatidae, Vinciguerria attenuata 



The swimbladder taken for sectioning came from a fish of standard length 32-5 mm. The sac was well 

 expanded, the major and minor axes measuring 7-5 and 2-5 mm. The gas-gland was flattened and 

 well displayed, appearing much like that of the fish described on pp. 7-9. 



Over the roof of the rear part of the swimbladder the walls measured about 20// in thickness. 

 There is a thin tunica externa formed of fibres running round the sac; then comes the reticulate 

 submucosa which takes up most of the wall thickness. Near the rete mirabile the walls are padded 

 • with a voluminous submucosa measuring about 250// in thickness. The swimbladder is lined with 

 a filmy pavement epithelium. 



Vinciguerri nimbaria 



The swimbladder described on p. 9 was used in the preparation of serial transverse sections. It is of 

 particular interest in that the sac was much contracted, but not collapsed, while the gas-gland formed 

 a thick bunched-up mass. 



The swimbladder is completely invested by a layer of peritoneum bearing many melanophores 

 and formed of close-set fibres. The thin tunica externa is made up of circular fibres, while the very 

 thick submucosa varies from 50 to ioo/< in width. Owing to the relaxation of the sac, the fibres of the 

 submucosa have been thrown into a series of undulations (see also p. 65). 



Cyclothone 



During the larval phase, the species of Cyclothone have a gas-filled swimbladder, which regresses and 

 becomes invested with fat after metamorphosis to the adult. The development of the latter condition is 

 dealt with in another section (pp. 65-68). Here we deal with the wall structure of the larval swimbladder. 



The species of Cyclothone studied by Nusbaum-Hilarowicz (1920) was probably braueri and not 

 signata (see p. 18). It is evident from his plate VIII that the swimbladder was in the larval condition, 

 although fig. 3 shows a trace of the regular reticulate tissue (which holds the fat) at one end of the sac. 



He found a thin but tough outer layer (tunica externa), consisting of long fibres that encircle the 

 sac followed by another thin layer of rounded and branching cells. The next and median layer (sub- 

 mucosa) was very voluminous at the front and rear sections, but quite thin in the middle region. This 

 was formed of fibrillar and loosely compacted tissue. The inner epithelial layer formed the gas-gland 

 at the front of the sac. 



Transverse sections through the swimbladder of a Cyclothone braueri of standard length 26-5 mm. 

 revealed much the same tissue structure. The swimbladder has a length of about 3 mm. and around 

 the gas-gland the walls have a thickness of from 100 to 200//. 



A 10-mm. larval stage of C. pygmaea from Thor station 144 was also sectioned. There is little that 

 need be added, except to mention that the sac is completely invested with a densely pigmented 



