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



is the curious apparatus spoken of in this Report as the coupling-spines.^ Among the 

 Gammarina occasionally these spines are numerous ; among the Hyperina there are rarely, 

 normally perhaps never, more than two to each peduncle. In both groups they are 

 clearly spines that have been modified to serve one and the same purpose, namely, to hold 

 the peduncles together for the swimming-stroke. For this purpose the apex of each spine 

 is blunted and has backward directed teeth, the edges also often having a retroverted ser- 

 rature, so that the spines of each pair of peduncles can be interlocked. That both groups, 

 notwithstanding their otherwise extremely divergent forms, should so universal^ possess 

 these coupling-spines, is surely a note of common ancestry. It is also easy to see that 

 two quite simple spines in this position might be of some service for the object in view by 

 the effect of mere friction, while natural selection would be ready to avail itself of any 

 variation in the direction of the roughening of the spine, until the strongly serrate edges 

 and dentate apices had been at length evolved. In the branches of the pleopods we find 

 another note of community of origin for the two groups above mentioned. Besides the 

 obvious similarity which these branches display in almost aU the genera and species, they 

 have in common the less easily noticed feature of carrying one or more cleft spines^ on 

 the inner margin of the first joint of the inner branch. To this there are only rare excep- 

 tions, and those, perhaps, not difficult to explain. Throughout the Hyperina it appears 

 that the joint in question never has more than one such spine, while in the Gammarina 

 the number varies. The object served by these spines is no doubt similar to that of the 

 coupling-spines. One arm of the cleft apex has a subtermiual expansion, and the other 

 arm is internally roughened or serrulate. By these contrivances a pair of the spines 

 lying crosswise helps to keep together the branches of the pair of pleopods, and so to add 

 force to the swimming-stroke. But these spines with cleft terminations have plumose 

 shafts, and are evidentlj^ plumose setae modified for a special purpose. Indeed, in some 

 species, in which the pairs of cleft spines are numerous, some of them show a gradational 

 form combining the flexibility of the seta with the cleft termination of the spine. 



Another example of gradational forms is exhibited by the maxillipeds of the 

 Gammarina. The outer plates of these organs are commonly fringed with an apparatus, 

 parts of which may be distinguished as respectively, teeth, spines, and setse, yet the teeth 

 pass into spines, and the spines into seta3 by gradations so minute, that the practical 

 difficulty arises in description of determining how many of these little appendages ought 

 to be grouped under one name, and how many under another, yet no one would dream 

 of interchanging the names of the two extremes of the series, the tooth and the seta. 



In classifying the families of the Amphipoda within the principal divisions, not a few 

 difficulties are encountered. AVe may attempt to place side by side those which in the 



' Described and figured by G. 0. Sars in his account of "Gammarns neglectiis, Lilljeliorg," Hist. Nat. Crust, d'eau 

 douce de Norv., p. 53, pi. v. fig. 8, "Opines particulieres," and indicated by S. I. Smith in his figure of Cerapits tahularis, 

 Say, Trans. Connect. Acad., vol. iv. pi. ii. a, fig. 5, but not, I think, alluded to by any other writers. 



2 "Soie particuliere i\ bout bifurqu^," Sar.=, loc. cit., fig. 8. 



