i 44 DISCOVERY REPORTS 



Mysidetes crassa Hansen, 191 3 



(Fig. 33 H) 

 1913 Mysidetes crassa Hansen, p. 18, figs. pi. 11, ya-g; pi. in, la-c. 



Occurrence : 



St. 51. 4. v. 26 (day). East Falkland Is., 105-115 m., 1 adult <$, 10 mm. 



St. 371. 14. iii. 30 (day). South Sandwich Is., 99-161 m., 2 ?? with well developed oostegites, 3 badly damaged 



(doubtful). 

 St. WS 213. 30. v. 28 (day). North of Falkland Is., 249-239 m., 1 adult $, 9-8 mm. 

 St. WS 226. 10. vi. 28 (day). North-west of Falkland Is., 144-152 m., 1 adult $, 10 mm., and 1 small juv. 



(damaged). 

 St. WS 773. 31. x. 31 (day). Patagonian Shelf, north of Falkland Is, 291-298 m., 1 $, broken in two pieces. 



Remarks. This species was founded by Hansen on a single immature female, which was captured 

 by the Swedish Antarctic Expedition (1901-3) off the Falkland Islands at a depth of 40 m. It has not 

 been recorded since. I am now able to add a few details to Hansen's description because the Discovery 

 collection contains adults of both sexes. 



The general form, proportions and shape of the rostrum, antennae and appendages of these speci- 

 mens agree very closely with the type. The shape and proportions of the uropods and telson also are 

 typical, but I find certain differences in the armature of these organs, which I ascribe to the immaturity 

 of Hansen's specimen. In the type only the distal half of the lateral margins of the telson is armed with 

 nine spines, each increasing regularly in size towards the apical lobes. These are armed with an 

 outer strong long spine and an inner shorter one. Only the proximal half of the cleft is armed with 

 small teeth, the distal half being naked. I give (Fig. 33 H) a figure of the posterior end of the adult 

 female from station 226. In it, the lateral margins of the telson are armed near the base with three 

 minute spines. These are followed by a small unarmed gap and then by a close row of 17-19 stronger 

 spines, which increase regularly in size towards the apex. Closer examination reveals that in many of 

 the spaces between these spines, there is a single minute spine, especially in the proximal half of the 

 spine row. Each apical lobe is armed with a long strong spine flanked by two, almost equal, shorter 

 spines on its inner side. The cleft is armed throughout the whole of its length with a close row of even, 

 fine teeth. The inner margin of the endopod of the uropod is armed with a close row of regularly- 

 graduated spines, which are very slender and relatively long distally, extending almost to the apex 

 (Fig. 33 H). Hansen did not mention any armature on the endopods of the uropods in his description 

 nor did he show any in his figure. These spines in the Discovery specimens are very slender and 

 difficult to see, but it seems unlikely that Hansen would have overlooked them had they been present 

 in the specimen he examined. 



Tattersall (1923, p. 287) recorded that in juvenile specimens of M. posthon the lateral margins of the 

 telson are armed with a few spines at the base followed by an unarmed portion and then with a close 

 row of spines which extend to the apex. In older animals the unarmed gap in the armature disappears, 

 so that there is an unbroken row of close spines from the base of the telson to the apical lobes. 

 Smaller spines appear between the older larger ones and in adults the typical arrangement in series 

 has been established. 



I suggest that the same development may occur in M. crassa. Although the specimen which I have 

 figured is apparently sexually mature, it may well be that growth continues after sexual maturity has 

 been reached. Larger animals may be found in which there is no gap at all in the armature of the 

 lateral margins and in which the slight tendency to an arrangement of the lateral spines in series has 

 proceeded further and in which spines have developed along the whole of the margins of the apical cleft. 



