18 DISCOVERY REPORTS 



In the 33-mm. fish the lumen of the sac was not entirely obliterated and the investment of fat was 

 relatively less voluminous, there being a padding of this tissue at the front and rear parts of the sac. 

 On the floor of the sac was a regressed gas-gland. The artery to this structure was traced backwards 

 to the dorsal aorta, while the vein runs to the bulbous part of the kidney, where it may join the renal 

 venous system. 



The 26'5-mm. fish has no fatty investment (and it is preserved in formalin) the thick-walled sac 

 measuring about 3 mm. in length. There is a heart-shaped gas-gland on the floor of the anterior half 

 and this receives a long rete mirabile (about 3-5 mm. in length) which enters the posterior end of the 

 organ. The microscopic structure of this swimbladder will be described in a later section (p. 66). 



Text-fig. 8. Fat-invested swimbladder of Cyclothone braueri. bk, bulbous, posterior part of kidney ; ft, fatty investment of 

 regressed swimbladder; oy, ovary; pv, pelvic fin; rsb, regressed swimbladder. ( x 18-9.) 



As the larvae of Cyclothone braueri have a gas-filled swimbladder, it is clear, as previously men- 

 tioned, that the organ gradually regresses and receives a thick coating of fat during the adult phase. 



Nusbaum-Hilarowicz (1920) studied the histology of the swimbladder of a species of Cyclothone, 

 under the name C. signata, but in view of Jespersen's and Taning's (1926) critical work, the species 

 is most likely to be C. braueri. The figures and description show that the swimbladder of this fish was 

 in much the same developmental phase as the 26-5 mm. individual described above. 



Cyclothone livida Brauer 



St. 3094, 44 20' N., i6°49'W., 21. v. 54, TYFH, i50o(-o)m. Standard length of two fishes examined 55 and 

 37 mm. 



In both these individuals the swimbladder has much the same position in the body-cavity as that 

 of C. braueri. It is sausage-shaped, having a length of about 5 mm. in the larger fish and about 

 4 mm. in the smaller one. The middle diameter in both is about 1-5 mm. 



Under the layer of black peritoneum that invests the swimbladder come the silvery or faintly 

 golden globules of the fatty tissue. This entirely fills the swimbladder and, as in C. braueri, the fat 

 droplets are held in a rather wide meshed reticular connective tissue. In the middle of the swim- 

 bladder is a cream-coloured body receiving blood-vessels entering the posterior end of the organ. 



The swimbladder of the larger fish was taken for transverse sectioning and the following extra 

 details of structure seen. At its posterior entry and for most of its course to the regressed gas-gland, 

 the blood system consists of six small vessels bound closely together. Three arterioles each have an 



