386 A HISTORY OF RECENT CRUSTACEA 
and fifth joints are, Hansen says, in this species firmly 
fused. Of the swimming-feet he observes that the 
feathered setze are attached a little within the margin, to 
obtain support when displayed for swimming, and to have 
a resting-place when folded away between the strokes. 
Eight new species are named and described by Beddard 
from the Challenger explorations, all from considerable and 
some from very great depths. | 
Ilyarachna, Sars, 1869, *‘ mud-spider,’ was named in 
place of the preoccupied Mesosténus, Sars, 1864, meaning 
‘narrow in the middle.’ The body is suboval, but deeply 
incised behind the fourth segment of the pereeon. The 
second antenne are long; the mandibles have a molar 
tubercle, and either have a small three-jointed ‘ palp’ or 
are without one. The first limbs of the perezon are not 
elongate, the second usually more robust than the rest, the 
third and fourth in general very elongate, the fifth and 
sixth natatory, with an unguiform seventh joint, the 
seventh long and slender, with the joints scarcely flattened, 
the nail long and curved. The uropods are simple, lying 
close to the pleon. Sars has described five Norwegian 
species, of which the first was Ilyarachna longicornis. 
Ilyarachna quadrispinosa, Beddard, was brought by the 
Challenger from Kerguelen. 
Desmoséma, Sars, 1864, ‘a chain-like body,’ has the 
segments marked off by constrictions which are commonly 
deep; the three hinder segments of the pleon are larger 
than those which precede. The second antenne are shorter 
than the body, slender in the female, robust in the male. 
The mandibles have a dentate cutting-edge, spine-row, 
molar, and usually a three-jointed ‘ palp,’ ending in a long 
unguiform spine. The first four pairs of limbs are short 
and robust, the last three natatory, but with the fifth and 
sixth joints not greatly expanded, edged with spines that 
are flattened but not plumose; the seventh joint is narrow 
and stiliform. The uropods are simple, two-jointed, the 
last joint being the larger. Sars has described four Nor- | 
wegian species, Desmosoma lineare, armatum, aculeatum, and 
tenuimanum, cf which aculeatum has no mandibular ‘ palp.’ 
