14 BRITISH COPEPODA. 



organs in Ectinosoma being excessively slender, and 

 almost linear in form. 



THE APPENDAGES OF THE THOEAX are, in their simplest 

 form, five pairs of swimming-feet, each foot consisting 

 of a 2-jointed base and two 3-jointed branches, but the 

 number of joints, though never exceeding three, is 

 within that limit, subject to a good deal of variation. 

 The second, third, and fourth pairs are in almost all 

 cases constructed as simple swimming-feet, without any 

 other function, but the first pair, not unfrequently (as 

 in most Harpacticidce) has one of its branches con- 

 verted into a clawed prehensile limb, and the fifth 

 pair shows very extensive modification in almost every 

 case, often differing very considerably in the two 

 sexes, and sometimes being very much reduced in 

 size, or even altogether wanting. Though, as has 

 been already said, the second, third, and fourth pairs 

 of feet are constructed simply as swimming organs, 

 and present no peculiarities of form, there are some 

 exceptions to that statement. In Metridia armata the 

 inner branch of the second foot in the male has a very 

 remarkably excavated notch with spinous margin 

 (PI. LVI, fig. 20 &), and some foreign species, belong- 

 ing, probably, to two distinct genera, Metridia and 

 Undina, exhibit a similar structure,* sometimes in both 

 sexes. Again, in the genus Harpacticus the inner 

 branch of the second foot of the male has the middle 

 joint produced downwards into a strong spine, which 

 varies in character according to species, but, in some 



* These species will be described in the ' Report on the Copepoda ' 

 taken during the voyage of H. M.S. " Challenger." 



