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



inner and posterior side of the third pair of pereiopoda, and protected by hairs around 

 the orifice, exposed when the leg is directed backwards, and hidden when it is thrown 

 forwards, being protected by the anterior margin of the thelycum, which lies between the 

 penultimate pair as a flattened plate, which is pointed forwards. A similar plate, 

 reversed in form, with a carinated ridge traversing the median line, stands between the 

 ultimate pair. It is absent or reduced to a minimum in the male, its place being 

 occupied by the enlargement of the coxal joints (PL XL V. fig. 2) of the posterior pair of 

 pereiopoda and the protuberance that carries the vas deferens, the slit-like aperture 

 for which is narrow. These organs almost meet in the centre, and a narrow tooth- 

 like process projects forwards from them on each side, and rests against the coxa of the 

 preceding legs. 



In the male the first pair of pleopoda (p) has the basisal joint transversely triangulate ; 

 the inner margin near the base carries a small and rigid process, from near the root of 

 which a large, foliaceous organ, the jDetasma, arises and spreads itself out until it reaches 

 its corresponding fellow, to which it is linked, in the adult, by a series of small 

 cincinnuli, thus forming a veil or curtain across the ventral surface. This pair of 

 pleopoda carries a long, two-jointed branch, the first joint of which is simple, and the 

 second multiarticulate. 



In the female these pleopoda are similar, but the inner branch, which forms the 

 petasma in the male, is reduced to a rudimentary condition. Between these pleopoda, 

 in the median line, is a long and laterally compressed tooth-like process. 



The second pair of pleopoda is biramose, the anterior branch being small, almost 

 rudimentary, in both sexes, but in the male there springs from its base a two-jointed 

 complementary appendage that is wide, flattened, and capable of being compressed 

 closely against the anterior surface of the posterior branch. The third and succeeding 

 pairs of pleopoda have the anterior or smaller branch gradually increasing in size until 

 in the fifth pair it is subequal to the posterior branch. 



The sixth pair forms the outer branches of the tail-fan ; the inner branch is markedly 

 ovate, strengthened in the median line by two longitudinal ridges, one of which is con- 

 tinuous from near the apex to the articular process, but the inner, or that nearer the telson, 

 diverges and curves towards the margin near the root of the telson. The outer branch 

 is also ovate, but much longer than the inner. It is strengthened by three longitudinal 

 ridges, one along the outer margin, terminating nearly opposite the extremity of the 

 telson ; the second on the. outer side of the median line, with which it runs parallel 

 until towards the extremity, where it curves outwards and unites with the outer ridge 

 to make a strong, sharp tooth ; the third is in the median line and terminates at the 

 extremity of an imperfect diaeresis ; between this ridge and the second is a deep sulcus. 

 The rest of the plate is smooth, and its inner and distal margins as well as those of the 

 inner or smaller plate, are fringed with ciliated hairs. 



