304 TAdK SKOGSBERO 



graphischer Verhaltnissu", as AfSTKIN suggests. During February 1904 we are coucerued vvitli 

 an ascent to the surface of the water from a (U'pth of 205 in. (at Stat. S. 1), to 50 — ni. from 

 a depth of 126 m. (at Stat. S. 2) and to 30—0 in. from 78 m. (at Stat. S. 5); dming February 

 1906 there was an ascent to the surface of the sea from 673 m. (Stat. S. 7) and fi-om 116 m. 

 (Stat. S. 16). /\n ascent of this sort is certainly altogether too difficult for larvae which have 

 no power of swimming, even if the hydrographic conditions were very unfavourable! 



On account of these facts and the facts shown in the table worked out by me and 

 given on p. 354, it seems to me probable that, as has been stated above, this species copulates 

 during all parts of the year. 



It seems strange that Ph. (Ph.) ylobosa is so seldom met with in the plankton. This is 

 shown by the preceding literature as well. As instances I need only mention here that C. W. 

 S. AURIVILLIUS found this species only on a single occasion (during the month of July) in Baffin's 

 Bay (C. W. S. Aurivillius, 1896, p. 211) and that the same writer did not find this species 

 planktonically in Skager Rak, in spite of careful studies of the plankton of this sea diiring several 

 years; the samples were taken during all the months of the year (C. W. S. AURIVILLIUS, 1898). 

 I may also here mention the fact that I only found this species in Skager Rak and Cattegat 

 in a few out of about a hundred samples of plankton from January, May and July (cf. p. 358). 

 What is the cause of this phenomenon? 



This seems to be very difficult or perhaps it would be more correct to say impossible 

 to decide with certainty at the present time. The fact that all the samples of plankton in which 

 I found this species were taken during the darkest part of the night, while the greater part of 

 the samples which did not contain this species were collected during the day first led me to 

 assume that the copulation of this species took place principally during the night. This assump- 

 tion seemed also to be supported by the statements of preceding writers. Thus, for instance, 

 G. S. Brady writes, 1868 b, p. 464 of Ph. (Ph.) interfuncta that it was ,, taken abundantly in 

 ,the towing-net at Cumbrae, chiefly at night time". G. W. MtiLLER writes 1894, p. 14 with 

 regard to the Cypridinids: ,,Wenn man sie gelegentUch freischwimmend in der Nahe 

 der Kiiste gefimden hat, so handelt es sich dabei um ein zeitweises Aufsteigen, das vorwiegend 

 bei Nacht zu erfolgen scheint." 



The extracts given above from the plankton tables published by the „C o n s e i 1 p e r- 

 m a n e n t" show, however, with all desirable clearness that this explanation is not correct. The 

 finds included in these tables are distributed fairly equally over the twenty -four hours of the day. 

 It seems most probable to me that the explanation of this phenomenon is to be found, 

 first, in the fact that the planktonic period of each individual is very short and, secondly, that, 

 as in the case of termites and ants, with the wings of which G. W. MOller, as we have seen 

 above, has compared the long natatory bristles of the second antenna of this species — the 

 swarming individuals appear in flocks. An appearance in flocks would of course greatly de- 

 crease the chances of catching the species in the plankton nets. The idea that this species 

 appears in flocks during its planktonic period seems to be supported especially by the samples 

 from D. N. 9, 3. V. 1903, S. 1, 16. II. 1904, Da. 4, 1. V. 1904 and Da. 20, 1. V. 1904. At 

 all these stations this species was found abundantlv or even very abundantly in a single sain])le 



