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



Structure of the antcnnulte in the latter forms by instituting a comparison with the cor- 

 responding limbs in the Copepoda. Thus, in the Harpactoidea we find that the anterior 

 pair of antenna3, which undoubtedly answer to the antennulas in the Nebaliida3, are 

 composed each of two rather distinctly defined divisions, a thicker proximal part and a 

 much narrower distal part, the former consisting pretty constantly of four articulations, 

 the last of which forms anteriorly a lateral expansion bearing a slender, ribbon-like 

 appendage, the latter composed of a varying number of articulations but alwaj^s very 

 sharply marked off from the proximal part. There cannot in my judgment be any doubt 

 that the proximal part of the anterior antenn® in these Copepo^.1a answers to the anten- 

 uular peduncle in the Nebaliicte, and the distal part to the flagellum. The lateral 

 expansion of the last joint of the proximal part in the former is also well seen in the 

 Nebaliidse, and, moreover, in all probability the ribbon-like appendage affixed to that 

 expansion in the Haq:)actoidea is a homologue of the setose lameUa in the Nebaliidse. 

 Thus, all parts of the antennulse in the latter forms seem to be in fact represented in the 

 Copepod antennula, and the apparently abnormal numljer of joints in the peduncle as 

 compared with that in the higher Crustacea, will turn out to be quite normal in relation 

 to the Copepoda. 



Homology of the Antennas. — In comparing the antennas in the Nebaliida? mth those 

 in the higher Crustacea, we find at once their structure very diiferent from that in any 

 known form of the Podophthalmia, whereas they at first sight somewhat resemble the 

 lower antennfe in the Amphipoda. On closer examination, however, it is easily found 

 that they differ materially also from those in the last named grouj] of Crustacea by the 

 peduncle being only triartieulate, whereas in all known Malacostraca the number of joints 

 is far greater, generally amounting to five in all. Moreover, the peculiar geniculate 

 bend at the middle of the peduncle is rather different from what is generally met with 

 in the Malacostraca. I think we may also in the case of these limbs more properly 

 derive their structure from that met with in the Copepoda, especially those of the 

 Harpactoid group. In these Copepoda the posterior antenna; are, as is well known, 

 very small but composed of three distinctly defined segments, the last of which forms 

 with the preceding a strong geniculate bend, and it may readily be found, by comparison, 

 that these three segments together perfectly agree in form with the antennal peduncle 

 in the Nebaliida3. At the tip of the last joint, moreover, in some forms a small 

 imperfectly defined terminal joint may be distinguished, representing a rudiment of the 

 flagellum. The accessory branch generally found affixed to the middle of the second 

 segment is sometimes quite obsolete, whereby the accordance becomes still more pro- 

 nounced. The peculiar modification of the flagellum in the male of Nehalia, somewhat 

 similar to what is met with in the Amphipoda and Cumacca, does not seem to have any 

 more general significance, since in the nearly related genus Paranehalia there is 

 no difference whatever to be found in the antennse of the male and female, whereas it 



