SURVEY OF STRUCTURE 19 



associated venule. Close to the gas-gland there are twelve vessels formed no doubt by the forking of 

 each of the six vessels. The twelve vessels continue into the pear-shaped mass of the gas-gland. 



Besides these two fishes, dissection of several other smaller individuals has revealed transition 

 stages between the gas-producing swimbladder of the larvae and the fat-invested structure of the 

 adult (see p. 66). 



The microscopic structure of the swimbladder of the 55-mm. fish will be described in a later 

 section (pp. 66-67). 



Cyclothone microdon (GiAnther) (Text-fig. 7 A, b) 

 St. 239, 46 56' 00" S., 46 03' 00" W., 2. vi. 27, N 450, i35o-io5o(-o) m. Standard length of fish 52 mm. 



As in other adult Cyclothone, the fat-invested swimbladder of C. microdon is slung in the rear part 

 of the body-cavity, the backward end lying just before the enlarged posterior part of the kidneys. 



In the middle of the blimp-shaped mass of fatty tissue is a small pear-shaped, cream-coloured 

 body, receiving blood-vessels that enter the posterior under-surface of the organ. A vein, which may 

 be an extension of the renal venous complex, runs forward from the bulbous posterior part of the 

 kidney. The origin of the artery that runs side by side with it could not be traced with certainty. 

 After entering the swimbladder these two vessels continue forward within the fatty tissue and spiral 

 round each other just before entering the pear-shaped structure. This consists of regressed glandular 

 cells and associated capillaries, which are invested and bound together with connective tissues. In 

 the above fish this structure measured 075 mm. in length and 0-3 mm. in greatest width, while in 

 another individual of the same size the length was 0-4 mm. 



Cyclothone acclinidens Garman 



John Murray Expedition, St. 95 Arabian Sea, 2-m. tow-net, 1400-m. wire. B.M. Reg. no. 1939.5.24. 115-250. 

 Standard length of fish 30 mm. 

 The swimbladder of this species has a very similar structure to that in the four species just described, 



Family Sternoptychidae 



Argyropelecus aculeatus Cuvier & Valenciennes (Text-fig. 9) 



St. 257, 35 01' 00" S., io° 18' 00" E., 24. vi. 27, N100H, mm. B.M. Reg. no. 1930.1.12. 367-8. Standard 

 length 23 mm. (3-5 x 2-5 mm.). 



The swimbladder of this hatchet fish is ellipsoidal in form and is slung in the upper part of the 

 body-cavity. At the posterior end of the sac is a massive rete mirabile. It originates at the level of 

 the long axis and extends under the sac over a length of 1-2 mm. Seen from below, the rete is acorn- 

 shaped, the bulbous head consisting of the first division of the artery and vein into finer vessels. 

 These then divide again into the thousands of closely intercalated capillaries that form the rest of this 

 structure. 



The gas-gland encircles a heart-shaped capillary bed which extends over the middle region of the 

 swimbladder floor. There are two glandular lobes, each consisting of a band of tissue with expanded 

 posterior and anterior regions (see Text-fig. o,b). 



Arteries and veins leave the front of the rete and run into the gas-gland. There are lateral vessels to 

 the posterior expanded parts of the gland and three central vessels running forwards along the mid- 

 ventral line. The middle vessel is a vein which forks to send a branch to each of the anterior expanded 

 parts. On either side of the vein is an artery, each of which follows the bifurcations to the gas-gland. 



The arterial supply to the capillary network comes from the artery to the rete mirabile, which 

 sends off a branch just before its point of entry. This branch runs forwards along the right side of 



3-2 



