STRUCTURE AND SYSTEMATICS 57 



The Miripinnati and most Anoplogastroidea resemble the stomiatoids in having retia that are formed 

 at the posterior end of the swimbladder, although both groups consist of euphysoclists, while all 

 stomiatoids are paraphysoclists. The transformation of a stomiatoid type of swimbladder, with its 

 unique vascular arrangements for the resorption of gases, to either the miripinnatous or anoplo- 

 gastrid form would seem most unlikely. It is thus evident that the stomiatoids are a well-defined 

 phyletic line of isospondylous fishes which, as the result of evolution in other directions, have left no 

 living descendants. 



Earlier discussion suggests this is also true of the deep-sea salmonoids. They are a compact, 

 specialized group, forming a morphological (perhaps also an evolutionary) series towards forms like 

 Opisthoproctns. However, the 'bauplan' of their swimbladder could very well have been derived 

 from the more generalized condition seen in the salmonids. 1 



te 



sm 





i 





i >>'.' ft... # 



' •-- 2j''« 



mm 



. mrm 



Text-fig. 3 1 . Semidiagrammatic representations of transverse sections through the swimbladder wall of (a) Salmo ; (b) Opis- 

 thoproctus. cp, capillaries ; ie, inner epithelium ; gg, gas-gland ; mrm, micro-rete mirabile ; sm, submucosa ; sml, layer of smooth 

 muscle ; te, tunica externa. 



In Salmo the swimbladder is supplied with arterial blood through branches of the coeliaco-mesen- 

 teric and intercostal vessels. Venous blood leaves through a vessel running forward to the portal 

 system and through others leading to the intercostal and gonadial veins. The capillary stem may here 

 and there form associations, but there is no regular development of retia mirabilia. The inner epithe- 

 lium over the forward part of the sac, consists of columnar cells, elsewhere of flattened or cubical 

 cells (Rauther, 1922). All that is required to turn such a swimbladder into the deep-sea salmonoid 

 type is the regular association of capillaries to form micro-retia mirabilia, the differentiation of the 

 inner epithelium to form gas-glands and, of course, the loss of the pneumatic duct. This may be 

 better followed in Text-fig. 31, which shows semi-diagrammatic, transverse sections through the 

 swimbladder walls of Salmo fario and Opisthoproctus soleatus. 



The lineages represented by the orders Iniomi, Cetunculi, Chondrobrachii and Miripinnati may 

 well have evolved from a common ancestor. Presumably this ancestral fish had advanced beyond the 

 level of organization of the primitive Isospondyli (in that its premaxillae excluded the maxillae from 

 the biting edge of the upper jaw and the mesocoracoid arch was lost from the shoulder girdle). The 

 orders Giganturoidea and Lyomeri may also belong to this evolutionary complex (Bertelsen and 

 Marshall, 1956). 



The only fishes in these orders with swimbladders are the Myctophidae, Neoscopelidae (Iniomi), 

 and the Miripinnati. The Myctophidae and Neoscopelns are considered to be rather closely related, 

 1 Fahlen (1959) has shown that there are (micro)-retia mirabilia in the swimbladder of Coregonus lavaretus. 



