REPORT ON THE CRUSTACEA MACRURA. 393 



gnathopoda; it consists of six joints, of which the first three are short and robust, the 

 meros is long and slender, the carpos long and cylindrical, and the propodos or terminal 

 joint is long, much more slender than the preceding, and strongly fringed with hairs. 

 The two succeeding pairs of pereiopoda are seven-jointed ; they are formed on the 

 same general type as the first, but longer and a little more robust, and terminate in a 

 minute chela of almost microscopic dimensions, which is all but hidden by the surrounding- 

 cilia. The fourth pair of pereiopoda in Kroyer's specimen is six-jointed, tolerably 

 robust, and furnished with long hairs on the three distal joints. In the Challenger 

 specimen, however, the fourth and fifth pairs of appendages are in only a young or 

 rudimentary condition, which can scarcely be the result of an immature stage, seeing that 

 in the male animal the prosartema is developed and the animal apparently possesses 

 functional power. I can therefore only assume that, after the animal has arrived at 

 maturity, the posterior two pairs of pereiopoda increase in value and importance without 

 ever attaining any functional power. The posterior pair is in a more feeble condition 

 than the fourth. 



The pleopoda are short and robust. The first pair in the female is single-branched 

 and simple; all the others are biramose. In the male the first (PI. LXIX., p,p) carries 

 on the inner side, near the middle, attached to a pedicle, a large, membranous petasma, that 

 is united in the median line with a corresponding one on the opposite appendage by a 

 series of small cincinnuli. The second pair of pleopoda (q) is biramose, and in the male 

 has the anterior or inner branch developed at the base into a strong lobe that is serrate 

 at the free extremity. The third pair is also biramose, and carries a lobe at the base of 

 the inner branch, but is not serrate. The succeeding pairs are biramose, simple, but 

 become shorter and more robust posteriorly. 



The sixth pair, which forms the outer plates of the rhipidura, has the outer branch 

 about one-third longer than the inner; the outer margin is armed with a small sharp 

 tooth, distant about one-third from the apex ; from this tooth the margin rapidly tapers 

 to the distal extremity, and is fringed with short, soft hairs, that lengthen a little as 

 they approach the apex, which is slightly truncated and furnished with three or four 

 long, ciliated hairs, continued in gradually diminishing size upon the inner margin 

 untd they approach the base of the plate. The inner branch or plate is narrower than 

 the outer, and gradually tapers from the base to the distal extremity; the inner and outer 

 margins are fringed with ciliated hairs that gradually increase in length from the base to 

 the apex. 



The telson is about half the length of the inner branch ; it is lobed on each side near 

 the base, and then gradually tapers to a slightly truncated extremity, the margin being 

 fringed with ciliated hairs that gradually increase in length from the base of the telson to 

 the apex. 



Sergestes arcticus, Kroyer, agrees in its general form with Sergestes atlanticus, 



(ZOOL. CHALL. EXP. — PART LII. — 1886.) Fff 50 



