SYSTEMATIC REPORT 143 



St. 1652. 23. i. 36 (day). Bay of Whales, 567 m., 1 imm. ?, 16-8 mm. 



St. 1660. 27. i. 36 (day). Bay of Whales, 351 m., 1 adult ?, 25 mm. 



St. 1872. 12. xi. 36 (day). Scotia Sea, 247 m., 1 imm. J, 11-4 mm. 



St. 1955. 29. i. 37 (day). South Shetlands, 440-410 m., 5 $<$, 4 ?$, largest 22 m., adult. 



St. 1957. 3. xii. 37 (day). South Shetlands, 785-810 m., 2 $S, 19 mm., 1 ?, 18 mm. Colour note 'Crimson, eyes 



golden'. 

 St. WS 219. 3. vi. 28 (day). North of Falkland Is., 116-114 m., 1 <?, 14-4 mm. 

 St. WS 229. 1. vii. 28 (day). North-east of Falkland Is., 210-271 m., 2 Si, larger 17 mm. adult; 58 juv. up to 



14 mm. 

 St. WS 239. 15. vii. 28 (night). West of Falkland Is., 196-193 mm., 1 S, 16 mm. 



Remarks. The specimens which I have here referred to M. posthon show considerable individual 

 variation, but these are too inconsistent to admit of the grouping of the animals into separate species. 

 As is the case in many other species, specimens from the southern, colder waters are much larger than 

 those captured farther north and attain sexual maturity at a much larger size. Females of 15-16 mm. 

 taken at station 45 off South Georgia have large, fully developed oostegites, while specimens of over 

 1 8 mm. from the Bay of Whales are quite immature with small oostegites only just beginning to develop. 



There is considerable variation in the size of the eyes and in the amount of uptilting of the lateral 

 margins of the small rostral plate. It may be that the specimens, which I have referred to M. posthon, 

 represent in fact more than one species, but no two specimens appear to be absolutely alike and I think 

 it better for the present to regard the differences as examples of individual variation. 



Distribution. M. posthon has a circumpolar distribution and has been taken, often in considerable 

 numbers, by all the principal South Polar and Antarctic Expeditions. The most northerly latitude at 

 which it has been taken is from near Kerguelen Island at 'about 49 S.' (Illig, 1930, p. 488) and the 

 most southerly from the Bay of Whales, 75 56-2' S. in the Discovery collection. It would appear to 

 be gregarious in its habits, especially when young. 



Mysidetes kerguelensis (Illig), 1906 



1906a Metamysidella kergnele?isis Illig, p. 210, figs. 

 1930 Mysidetes kerguelensis (Illig), p. 472, text-figs. 



Occurrence : 



St. MS 68. 2. iii. 26 (day). East Cumberland Bay, 220-247 m., 14 adult Si, largest 8-4 mm., 3 adult $$ (2 with 



large full brood sacs), 8-6 mm., 23 juv. 6*<J, 16 juv. ?$, 6 very small juv. 

 St. 39. 25. iii. 26 (day). East Cumberland Bay, 179-235 m., 3 adult SS, largest 8-2 mm., 2 juv. SS, 6 mm., 



1 adult $, 8-4 mm. 

 St. 142. 30. xii. 26 (day). East Cumberland Bay, 88-273 m -> 2 imm - <Sd> ^^T 6 mm -> l adult ?> 8- 4 mm -> 2 J uv - 



Remarks. The Discovery specimens agree very closely with Illig's description and figures, but 

 appear to be adult at a smaller size than his types. The most remarkable feature of this species is the 

 enormous development of the male genital organ. In the immature specimen of 6-4 mm. from station 

 142 it extends as far forward as the mouth, but in all the males of over 8 mm. it stretches forward 

 quite considerably beyond the distal margin of the antennular peduncle. In the specimen of 8-2 mm. 

 from station 39 the genital organ measures just over 5 mm. 



The specimens which were taken at station MS 68 are all very dark in colour, but those from the 

 other stations are opaque and quite colourless and have no pigment in the eyes. This gives them 

 a peculiar dead appearance and makes the animals easy to pick out when sorting. 



Distribution. The types were taken off Kerguelen Island and there have been no other records 

 until the present one from Cumberland Bay, South Georgia. 



