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1. Ilyarachna longicornis,
(PI. LIX.)
G. 0. Sars.
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
2 species by the smoothness of the cephalon and the anterior segments
mesosome. It is also of far inferior size, and is less robust of form.
from the other
of the
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).