TETRAPLATIA VOLITANS 137 



to the direction of the prevailing winds, which, at this season, blow offshore, displacing surface water 

 and probably causing an upwelling of deeper water. 



Elsewhere Tetraplatia has been taken in all seasons except in January (Hand, 1955) and ' Discovery II ' 

 has taken it all the year round except in January and October. 



So long as Tetraplatia appeared to be of seasonal occurrence in the plankton, it was thought to 

 have a fixed benthic stage, but since records from several open ocean stations were reported by 

 Leloup (1935), it has been rightly considered to be wholly pelagic. There is a suggestion, as in other 

 wholly planktonic animals, that the young stages of Tetraplatia are found in the upper surface layers. 

 'Carnegie' took six specimens o-5-o-8 mm. in length in a surface haul at St. 93, and at St. 33^ 

 H.M.S. 'Research' took a specimen of 0-9 mm. in the upper 100 m. Another specimen of the same 

 size was taken at 'Discovery II' St. 1913 (250-160 m.), but below this depth no specimen under 

 i-o mm. in length was taken in any of the hauls. 



HYDROGRAPHIC CORRELATIONS 



Fortunately hydrographic observations were made at most of the stations where closing-nets were 

 operated and these are summarized in Table 3. The data are plotted on temperature-salinity charts, 

 similar to those recommended by Pickford (1946) for the analysis of hydrographic correlations 

 (Figs. S-7). 



Fig. 5. Distribution of Tetraplatia volitam in relation to temperature and salinity in the South Atlantic. Each specimen is 

 plotted according to the recorded sahnity and temperature for the depth nearest to the mid-depth of the closing-net at that 

 station. No records of Tetraplatia have been used unless temperature and salinity reading have been available for that 

 particular station. 



