STRUCTURE AND SYSTEMATICS 53 



Argyropelecus. The gas-gland surrounds the resorbent area which is on the floor of the swimbladder. The form 

 of the swimbladder is ellipsoidal but approaches that of a sphere. 



Polyipnus. The gas-gland is bilobed and the resorbent area lies between the lobes. In the adult the swimbladder 

 is invested with fat. 



It will be seen that certain swimbladder features, particularly the form of the gas-gland and its 

 position relative to the resorbent area, can be used as diagnostic features in certain of the genera. 

 Perhaps the associated circulatory system could also have been used; for instance, in the genera 

 Vinciguerria, Maurolicus and Polltchthys. But the circulatory system can vary considerably within a 

 genus and its species (see Godsil and Byer's (1944) study of the tunnies). Until more data are avail- 

 able it would seem best not to use the arrangement of blood-vessels as taxonomic characters. 



Lastly, this survey has not been wide enough for much to be said on the distinguishing of species 

 of deep-sea fishes by means of individual swimbladder structures. In one genus, Gonostoma, the 

 three species are very unlike in this respect; I have already written (Marshall, 1950, 1954, 1955; 

 Jones and Marshall, 1953) that G. denudatum has a normal type of swimbladder, that in G. elongation 

 the swimbladder regresses and becomes invested with fat, and that G. bathyphilun has no swimbladder. 

 Differences of this order would seem, at first sight, to call for separate generic status for these three 

 species. But reference to the papers just mentioned will show that there is a correlation between the 

 degree of development of the swimbladder and the vertical distribution of each species. There seem 

 tp be parallel correlations in eye-structure, coloration (Marshall, 1954) and in other features (pp. 99- 

 100). As closer study of such correlations is in progress, review of the status of the genus Gonostoma 

 will be left until this work is completed. At present, however, there seems little justification for 

 changing the present classification. 



Suborder Salmonoidea 



The design of the swimbladder in the deep-sea salmonoids is very unlike that of the stomiatoids. The 

 individual features are as follows: The swimbladder is euphysoclistous 1 (see footnote on p. 50) and 

 the blood-vessels forming the retial system break up into numerous micro-retia, each consisting of 

 a relatively small number (less than 50) of arterial and venous capillaries (or arterioles and venules). 

 The gas-gland is diffuse rather than compact (Text-fig. 30 b). 



The deep-sea salmonoids undoubtedly form a compact, natural group within the suborder Sal- 

 monoidea. Gunther (1866) placed Argentina and Microstoma in the Salmonidae and Brauer (1906) 

 regarded Winter ia and Opisthoproctus as other members of this family. Trewavas's (1933) anatomical 

 study of Opisthoproctus properly revealed its salmonoid affinities, while the later work of Chapman 

 19420, b, 1943, 1948) clearly established the relationships between the above genera and Rhyncho- 

 hyalus, Bathylagus, Leuroglossus, Dolichopteryx and Macropinna. More recently, Bertelsen (1958) has 

 shown that Xenophthalmichthys is also an argentinoid fish, being closely related to Microstoma 

 and Nansenia. 



After Berg (1940) had introduced a new suborder (Opisthoproctoidei) for Opisthoproctus, Chapman 

 (19426) subscribed to this opinion, but proposed that all the other genera should also be included in 

 the new suborder (Berg puts them in the Salmonoidea). 



The swimbladder characters support Chapman's contention that Opisthoproctus should not be 

 placed apart from the other deep-sea salmonoids. To Chapman's (1942 a) list of characters for his 

 ' opisthoproctoid ' fishes may be added those given above concerning the swimbladder. Furthermore, 

 the similarities in the design of the swimbladder are so striking that some support is given to Hubbs's 



1 In Argentina, Glossanodon, Microstoma and perhaps in Nansenia the posterior part of the swimbladder is thin-walled 

 and functions as the resorptive part of the sac. In Opisthoproctus and Winteria this part of the sac may also have this function, 

 but the material available was not sufficient to confirm this suspicion. 



