98 THE VOYAGE OF H.M.S. CHALLENGER. 



the cephalothorax, very narrow, and equal in diameter throughout its entire length, five- 

 jointed, the various joints nearly equal in length. Anterior antennae (fig. 1) composed 

 of thirteen joints, of which the second and third and the last four are very short and 

 nearly equal, the fourth, fifth, and ninth being the longest; the proportionate lengths of 

 the joints are approximately as follows : — \ \ \ \ \ \ \ \ 9 7 *° " l \ " The limb bears 

 about twelve very long and slender seta?. The maxilla (fig. 4) has a stoutly armed 

 prehensile portion and a well-developed palp, to which is attached a trisetose branchial 

 (?) plate. The first pair of swimming feet (fig. 7) has both branches three-jointed, 

 the outer branch bearing a marginal series of small aculeated spines ; the branches of the 

 following three pairs are also three-jointed (fig. 8), but have no marginal spines. The 

 terminal spines of all the feet are excessively long and slender, and are finely pectinated 

 on the outer margin. The fifth foot (figs. 1, 9, 10) consists of a small tubercle, to 

 which are attached two long biarticulate setae. Caudal segments slightly divergent, 

 nearly as long as the preceding abdominal segment, each bearing six plumose setae, 

 four of which are about as long as the abdomen; one of these arises from near the base 

 of the external margin ; two others — the innermost and outermost of the apical series — 

 are much shorter than the rest. 



Dana has described three species of Oithona, Claus two, and Boeck two, but none of 

 these seem quite to agree with the form which occurs in several of the Challenger 

 gatherings. The animal, however, is so fragile, and so difficult of examination, that it is 

 quite possible that errors of observation may have affected the descriptions both of 

 myself and other authors. But, so far as I can make out, the antennae of this species are 

 certainly thirteen-jointed. Dana assigns only seven joints to those of all his species, and 

 I cannot but think that he must have failed to observe them correctly. Claus's species 

 have respectively ten and twelve joints. The present species approaches most closely to 

 Oithona spinirostris, Claus, and perhaps to Oithona setiger, Dana, but the differences are 

 too considerable to allow of our safely referring it to either of these. Though examples 

 of Oithona were noticed in very many of the surface-gatherings of the Challenger, 

 I have not been able to recognise differences requiring their reference to more than one 

 species. The form doubtfully referred in the general list of species (p. 5) to Oithona 

 spinirostris, Claus, I cannot on re-examination distinguish from the present species. 



Specimens which I refer to this species were found in the produce of the surface-net 

 from the following localities : — Off the south of Papua ; off Port Jackson, Australia ; west 

 of the Philippine Islands (Station 206) ; in Hilo Harbour, Sandwich Islands, abundantly; 

 in lat. 42° 32' S., long. 56° 27' W. (Station 318); lat. 32° 24' S., long. 13° 5' W. (Station 

 335); lat. 12° 16' S., long. 13° 44' W. (Station 341); lat. 9° 43' S., long. 13° 51' W. 

 (Station 342) ; in all the Stations between lat. 3° 10' N., long. 14° 51' W.; and 

 lat. 10° 55' N., long. 17° 46' W. (Stations 348-352); and in lat. 26° 21' N., long. 

 33° 37' W. (Station 353). 



