J g DISCOVERY REPORTS 



1 000-1500 m. level. When the results are plotted on a chart, many of the records are seen to be from 

 localities where vertical displacement of water masses is likely to take place. 



Table 2. Vertical distribution of the specimens from the closing-net hauls of R.R.S. 'Discovery 11' 



Depth in metres Number of specimens 



I 



O-IOO 



100-250 

 250-500 



500-750 



750-1000 

 I 000- I 5 00 

 1500-2000 



LENGTH OF TETRAPLAT/A IN MM. 



2 3 4 5 6 7 



5 

 8 

 8 



4 

 I 



OCSD 

 O 



-■ 1 •- 



—> 1 ' — 



o o 

 00 



000 



-r 



00 O o 



DAYLIGHT 







100 



2; 200 



Z 300 

 Z 400 



g 



H 500 

 5 600 



a: 



fe 700 

 '-' 800 



1 1000 



LU 



> 1100 - 

 1200 

 1300 



LENGTH OF TETRAPLATIA IN MM. 



12 3 4 5 6 7 



T ' 1 • 1—— 1 1 T 



-r 



• • 



• • 



NIGHT 



_l I L. 



Fig- 3 



Fig- 4 



Figs. 3-4. 



Vertical distribution of Tetraplatia volitans. All records below 100 m. are from the closing-net hauls of 

 R.R.S. 'Discovery IT: fig. 3, by day; fig. 4, by night. 



The catches of the closing-net hauls have been plotted in Figs. 3 and 4 for any indication of diurnal 

 vertical migration, but it must be remembered that the catches are from different parts of the world 

 and the marked migration reported by Hand from California is perhaps masked by different local 

 conditions. By day Tetraplatia was found at the surface down to 500-750 m. with a slight pre- 

 ponderance of young stages in the upper 100 m. At 100-250 m. specimens of 0-9-5-4 ^^- ^^ length 

 were found, but below this depth, between 250 and 500 m., only individuals of 2-6-5-7 mm. occurred, 

 and still deeper, at 500-750 m., specimens of 3-5-4-7 mm. were taken. There is no positive indication 

 here (perhaps, as already mentioned, because of the varied localities at which captures were made) 

 that Tetraplatia tends to go deeper by day. 



Its vertical distribution by night is indicated in Fig. 4. Except for a small specimen (i-6 mm. in 

 length) from St. 2496 (1000-750 m.), there is a suggestion that the larger specimens are to be found 

 at the deeper levels. 



SEASONAL OCCURRENCE 



In the Mediterranean, Tetraplatia volitans was recorded only seasonally by the early investigators. It 

 was found from December to April at Algiers by Viguier (1885) and from January to the end of July 

 at Messina (Carlgren, 1909), and this seasonal appearance is probably due to local changes in hydro- 

 graphic conditions. In the Bay of Algiers the appearance of Tetraplatia in winter is probably related 



