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



foot-jaws (fig. 10) small, alike in both sexes, produced apically into a strong hook-like 

 extremity, and bearing on the inner margin several ciliated setae ; posterior foot-jaws 

 (fig. 11) elongated, three-jointed, forming a strongly clawed prehensile hand, the claw 

 larger in the male. First, second, and third pairs of feet with both branches three-jointed 

 (figs. 12, 13), inner branch of the fourth pair (fig. 14) quite rudimentary,, or replaced by 

 a single small hair. Fifth pair of feet wanting or excessively minute. Frontal eyes two, 

 each composed of a single, large, colourless, highly refracting lens, situated near the 

 base of the anterior antennae ; median eyes very small. 



The principal distinctive characters of this genus are the very large frontally situated 

 corneal lenses, the broad, cylindrical cephalothorax, the very narrow abdomen, consisting 

 only of two-joints and distinctly separated from the cephalothorax, the large, prehensile, 

 posterior antennae, the one-branched fourth foot, and the absence or very rudimentary 

 characters of the fifth pair. From the most closely allied genus (Saphimna) , the 

 cylindrical, non-complanate character of the cephalothorax and styliform build of the 

 abdomen distinguish it at a glance. 



But though the genus is at once and easily recognisable, the indication of specific 

 characters is a most perplexing task. From the large number of gatherings which have 

 come under my notice it would be easy enough to pick out a considerable number, — perhaps 

 half a dozen types, — which, were we to ignore intermediate forms, might serve for the 

 foundation of as many separate species. But there would still remain numerous 

 individuals, not precisely agreeing at all points with the types, but, allowing for slight 

 variations, referable indifferently to several species. The characters upon which, chiefly, 

 the species already described have been founded are, — the general outline of the hody, 

 position of the eyes, form of the two pairs of antennae and posterior foot-jaws, and of the 

 caudal stylets. I have not, however, been able to satisfy myself of the validity of many 

 of the so-called specific distinctions which have been based upon these variations, so that, 

 while not venturing to deny the possible existence amongst the Challenger collections 

 of more species than I have admitted, it seems to me more consistent with the present 

 state of our knowledge to allow a very wide margin for variation, and so to restrict very 

 considerably the number of specific types. I feel sure, for instance, that some of Dana's 

 species are founded upon characters belonging to immature or quite young forms, — the 

 form and proportions of the caudal stylets and the armature of the posterior antennae, 

 being notably, as I believe, variable with the growth of the individual. But without the 

 opportunity of observing accurately the growth and development of the animals, it is of 

 course impossible to speak with absolute confidence on these points. We must always 

 bear in mind the possibility of two or more species being mixed up in the same gathering, 

 and when these are very closely allied, and in various stages of development, it becomes 

 almoot impossible to unravel the knot. 



