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



Paratanais dimorphus, F. E. Becldarcl (PI. XVII. figs. 1-8). 



Paratanais dimorphus, F. E. BedJarJ, 1'roc. Zool. Soc. Loud., 1886, pt. i. p. 1 11'. 



A very large number of a small species of Tanaid were dredged off Christmas Harbour, 

 Kerguelen, which appear to me to belong to the genus Paratanais. Some of these 

 individuals presented the ordinary characters of the genus, but others were found to 

 differ in the elongation of the cephalothorax, and in the peculiar structure of the cheli- 

 pedes, which will be described more minutely immediately ; these latter specimens, how- 

 ever, judging from the greater development of the antennules, are males, while the 

 specimens with normal chelipedes appear to be as invariably females. For this reason, 

 and because the two series of individuals were of about the same size, and were obtained 

 at exactly the same locality, I am inclined to suppose that they are merely the males 

 and females of the same species, which is thus characterised by a marked sexual 

 dimorphism. I am bound to state, however, that the only mark of sex which was dis- 

 coverable in these specimens is the varying structure of the antennules j 1 but this is 

 perhaps sufficient in the absence of other data t<> determine the sex with certainty ; and 

 if these individuals are not respectively the males and females of the same species, it is at 

 least a very remarkable coincidence, that of one species only males should have been 

 obtained, and of the other only females. 



Description of the Female. — The female (fig. 2) presents no very remarkable 

 characters, and it would indeed be hard to define the species, if it were not for the 

 male which is so extraordinarily different. 



The female specimens measure about 3 mm. in length. 



The cephalothorax projects very slightly between the bases of the antennules, the 

 eyes are large and oval, and have the normal position and structure; the length of the 

 cephalothorax is greater than that of the three following free segments of the thorax. 



The first free segment of the thorax is very short, the second about half as long again ; 

 the third and fourth continue to increase progressively in length ; the fifth segment is 

 very slightly longer, and the sixth decidedly shorter than the fourth. 



The abdomen is rather longer than the last two segments of the thorax, the segments 

 of which it is made up are equal, with the exception of the terminal segment, which is 

 longer than the rest. 



The antennules are shorter than the cephalothorax ; the first joint is the longest, the 

 second and third are very much shorter and subequal, the terminal joint being if any- 

 thing shorter than the preceding. 



The antennae are a trifle shorter than the antennules, and are five-jointed, the three 



1 In several, but not in all, instances ovigerous lamella; were present in the specimens which, on account of the 

 antenna?, would he regarded as females. Similarly, in certain of the supposed males, I could plainly Bee the paired 

 genital apertures on the last segment of the thorax. 



