60 DISCOVERY REPORTS 



seem that the larval swimbladder is not sufficient to bring the specific gravity of the larva to that of the 



environment. 'Buoyancy tanks' are required as well. 1 



The swimbladder wall 

 Excluding the peritoneal investment, there are four main layers of tissue in the swimbladder wall : 

 an outer, closely-knit fibrous layer; a middle, loosely- woven, fibrous tissue layer; a layer of smooth 

 muscle fibres, and an inner epithelial layer. The outer layer is usually known as the tunica externa, 

 while Fange (1953) called the middle layer the submucosa, for in structure and position it is very like 

 the layer of the vertebrate gut that already has this name. As Fange (1953) has said, '. . .from an 

 embryological and comparative anatomical point of view the teleostean swimbladder is nothing else 

 than an isolated part of the digestive tube, although adapted to special functions '. The inner layer 

 consists of pavement epithelium, which is locallyjiifferentiated to form the gas-gland. 



The swimbladder lies outside the coelom, between the gut and the kidneys. It is retroperitoneal in 

 position, but it becomes invested in varying degrees with peritoneal tissue. While the dorsal edge 

 of the peritoneum usually runs along the lateral walls of the swimbladder, in some teleosts this limit 

 lies above the organ (Freunde, 1938). Between the peritoneum and tunica externa, there may be 

 a padding of loose, reticular, connective-tissue cells. Strands of connective tissue also run between 

 the roof of the sac and the outer coat of the kidneys, which lies close to the swimbladder wall. 



The peritoneal coat and the tunica externa are mainly formed of closely packed collagen fibres. 

 The submucosa is by far the most voluminous coating of the swimbladder wall and it usually consists 

 of a reticulum of loosely packed, collagen fibres within a gelatinous ground-substance (Saupe, 1939; 

 Fange, 1953). This layer is particularly thick under the gas-producing structures of the swimbladder. 

 Between the submucosa and the inner epithelium there is a layer of smooth muscle cells which I have 

 been able to identify in a number of the fishes considered in this report. 



From this general outline we may turn to closer consideration of the swimbladder wall in certain 

 deep-sea teleosts. 



Family Sternoptychidae, Sternoptyx diaphana 



The fine structure of the swimbladder wall of this hatchet fish has been described by Nusbaum- 

 Hilarowicz (1920), who divided the layers of tissue into three sections: outer, middle and inner. The 

 outer section consists of two layers of long connective tissue fibres, the fibrillar axes of the topcoat 

 being at right angles to those of the undercoat. From his plate VIII, fig. 9 it is evident that the 

 fibres of the undercoat describe circular paths about the long axis of the sac. The middle section is 

 a loosely-woven reticulum of fibres, which are delicately fashioned and form most of the wall- 

 thickness. The cavities between the fibres hold a homogeneous and transparent substance, which is 

 not stained by such dyes as haematoxylin or eosin. Nusbaum-Hilarowicz suggested that in the living 

 animal this substance is probably a serous fluid. Undoubtedly this layer corresponds to what Fange 

 (1953) called the submucosa, and he remarked that, '. . .the submucosa often has a jelly-like, half- 

 fluid consistency '. As is usual, the inner layer consists of flattened epithelial cells which are locally 

 differentiated to form the gas-gland. 



Argyropelecus 



The histology of the swimbladder wall of Argyropelecus hemigymnus was also described by Nusbaum- 

 Hilarowicz. There is an outer layer, presumably the tunica externa, consisting of long fibres with 

 elongated nuclei, which for the most part are set across the long axis of the sac. Underlying this 



1 It would be interesting to know whether pelagic fish larvae with a swimbladder have relatively smaller subdermal spaces 

 than those without this organ. 



