24 



OSCAR SUND 



[REP. OF THE "MICHAEL SARS" NORTH 



Sergestes pectinatus (Hansen) nomen novum. 



Sergestes Henseni (Ortmann) H. J. Hansen 1896 (partim). 



This interesting species was described by Hansen 

 (1896) from "National" specimens, but it is not, as he 

 believed, identical with the species described from the 

 same sample by Ortmann (1893) as Sergio, henseni. On 

 comparing Ortmann's and Hansen's descriptions it appears 

 without doubt that both authors have founded their de- 

 scriptions on specimens of two distinct (though closely 

 related) species. Ortmann described one species, for which 

 I propose to retain the name S. Henseni while Hansen 

 fixed his attention chiefly upon an example of another 

 species, for which I here propose the name S. pectinatus, 

 alluding to the peculiar structure of the mxp. 3 first seen 

 by Hansen. That both species were present in the sample 



in question is certain from the following passage in 

 Hansens description relating to the outer uropods (1896, 

 p. 959): " — — in the one specimen the ciliated part 

 occupies three-fifths, in the other almost four-fifths of its 

 length. In no other species have I met with any similar 

 variation in this feature, but it also exists in the larvee". 

 One of the specimens must have lost its maxillipeds, 

 otherwise the error cannot be explained. Hansen's de- 

 scription may be completed by the following particulars. 

 The rostrum is rudimentary, consisting of a horizontal 

 spiniform process from the frontal margin, its length at most 

 equalling one-fifth of the breadth of the cornea. The three 

 joints of the antennular peduncles are of nearly equal 

 length. The relative length of the hairfringed portion of 

 the outer uropods is about four-fifths (in one specimen 



42 43 



a b c 



Figs. 42 — 13. Sergestes pectinatus. 42) J 1 6, st. 45, y 300. petasma ( 3 %). 43) tf 5, st. 45, y 300. a) 6. joint of left 3. mxp. ( 4 °/i). 



b) 4. and 5. joint of same ( 15 A). c) 4. and 5. joint of right 3. mxp ( 13 /i). 



carefully measured 82 %). The petasma (see fig. 39) is 

 of a type different both from that found in the "robustus- 

 group" and from that of S. vigilax. 



A peculiar interest is attached to a specimen (d" 5) 

 from st. 45, y 300 m. w., which presents all the char- 

 acters of S. pectinatus, except in the left mxp. 3 , which 

 is shown in fig. 43. The sixth joint is subdivided in the 

 typical manner, but the "comb" is wanting, being re- 

 placed by an armature of spines resembling that found 

 in the mastigopus and in the adults of other species. 

 I am inclined to regard this abnormality as an instance 

 of atavistic regeneration pointing to the origin of S. pec- 

 tinatus as a mutation from a nearly allied form. The right 



mxp. 3 is of the typical pectinatus-shape, the fifth joint 

 being adorned with "comb" (the sixth joint is missing). 



S. pectinatus is a very small species, apparently not 

 exceeding C = 8 mm., (total length about 25 mm.), and 

 the sexes may be distinguished even in specimens about 

 10 mm. in total length. 



Size (C in mm.) 



1 



2 



3 



4 



5 



6 



7 



8 



Total 



Males 



2 



17 



3 

 14 



4 

 7 



5 

 10 



3 

 8 



2 



1 



15 



Females 



28 



Young 



33 







