136 

 1. Ilyarachna longicornis, G. 0. Sars. 



(PI. LIX.) 



Mesostenus longicornis, G. O. Sars, Om en anomal Gruppe af Isopodei-. Chr. Vid. Selsk. Forhl. 



1863, p. 8. 



Specific Characters. Body about 3 times as long as it is broad, with the 

 anterior division of uniform width throughout, and about as long as the 3 posterior 

 segments of mesosome combined. Cephalon with the dorsal face perfectly smooth, 

 lateral parts obliquely expanded. The 4 anterior segments of mesosome differing 

 but little in size, being produced on each side to short anteriorly-pointing lappets, 

 anterior edge distinctly elevated and very smooth. Fifth segment of mesosome 

 rather large, being scarcely narrower than the preceding segments, and very deeply 

 emarginated behind ; last segment scarcely more than half as long as the penulti- 

 mate one. Caudal segment longer than it is broad at the base, and gradually 

 tapering behind, tip obtusely pointed. Superior antenna? scarcely reaching to the 

 middle of the penultimate peduncular joint of the inferior ones, basal joint nearly 

 smooth, flagellum composed in female of 6, in male of about 12 articulations. 

 Inferior antenna? fully twice as long as the body and very slender, the 2 outer 

 joints of the peduncle scarcely spinous, flagellum about the length of the peduncle. 

 1st pair of legs rather slender, with the carpal and propodal joints of about equal 

 length; 2nd pair with the propodal joint almost as long as the carpal one; the 

 2 succeeding pairs exceeding the body in length. Natatory legs of the structure 

 characteristic of the genus. Uropoda with the proximal joint oblong oval in form, 

 and edged with about 14 plumose setse, distal joint scarcely half as long, cylind- 

 ric, clothed with very delicate bristles. Colour whitish, semipellucid. Length of 

 adult female about 3 mm. 



Remarks. The present species, being the first recorded, ought to be re- 

 garded as the type of the genus. It may be easily distinguished from the other 

 2 species by the smoothness of the cephalon and the anterior segments of the 

 mesosome. It is also of far inferior size, and is less robust of form. 



Occurrence. I first detected this form in the inner part of the Christiania 

 Fjord, at depths ranging from 30 to 50 fathoms, and have also subsequently 

 found it in several other places on the Norwegian coast, as far north as Tjoto, 

 on the Nordland coast. Its natatory movements are extremely rapid, and are 

 effected by abrupt jerks backwards. When slowly creeping upon the bottom, it 

 somewhat recalls a spider, by its slender, laterally-extended legs, and the deep 

 median constriction of the body. 



Distribution. Skagerak (Meinert). 



