131 



rather peculiar, and somewhat recalls the species of the genus Ischnosoma, but 

 in the anatomical details it approaches more nearly to the genus Desmosoma. 

 The coarse armature of the body will serve for at once distinguishing it from the 

 species of either of the 2 above-mentioned genera. 



Occurrence. — This species was first observed in the inner part of the 

 Christiania Fjord, at a depth of 15 — 20 fathoms. Subsequently I have found it 

 also in several other localities of the Norwegian coast, as far north as the Lofoten 

 Islands, but it seems everywhere to be very rare. Out of Norway, it has not yet 

 been recorded. 



Fam. 5. Munnopsidae. 



Characters. — Body, as a rule, shorter and more compact than in the 

 Desmosomidce, with the 2 divisions of mesosome very sharply defined. Eyes ab- 

 sent. Superior antennae with the basal joint remarkably large and broad, flagel- 

 lum generally well developed. Inferior antenna? very much elongated, and of 

 same structure in the two sexes. Oral parts on the whole normal, though the 

 structure of the mandibles is rather different in the different genera. 1st pair 

 of legs generally smaller than the others, but never subcheliform; the 3 succeed- 

 ing ones, as a rule, very much elongated and ambulatory in character. The 3 

 posterior pairs of legs very unlike the preceding ones, and pronouncedly natatory, 

 the outer joints being lamellarly expanded, and fringed with densely plumose setse. 

 Uropoda small, simple or biramous. 



Remarks. — In the restriction here adopted, this family is prominently 

 distinguished by the peculiar structure of the 3 posterior pairs of legs, which 

 constitute very powerful natatory organs, by which the animal is enabled to move 

 very rapidly through the water in a backward direction. This motion has not, 

 as in certain Desmosomidce. the character of an even run, but is effected by 

 sudden jerks, and is, on the whole, much more energetic, a fact easily accounted 

 for by the rather different structure of the natatory legs, and the densely plu- 

 mose setse with which the very much expanded outer joints are fringed. The 

 family comprises at yet 7 genera, 6 of which are represented in the fauna of 

 Norway, and will be treated °^ below, the 7t ^ genus, Acanthocope, having been 

 established by Mr. F. Beddard, to include 2 species from the Challenger Expedition. 



