GENERAL ANATOMY. 7 



motion is thus a succession of rapidly recurring jerks 

 corresponding with the separate sweeps of the antennas. 

 These organs vary greatly in length and in the cha- 

 racter of their setose armature. In the Harpactinidce, 

 as well as in most parasitic and semi-parasitic species, 

 the length of the anterior antenna falls short of, or at 

 any rate does not usually much exceed, that of the 

 first division of the body (carapace), while in the 

 Cyclopidce it often equals, or even exceeds, that of 

 the cephalothorax. In many Galanidce the length 

 of the antennae is still greater, not unfrequently 

 exceeding sometimes very much exceeding that of 

 the entire animal. There is, however, no instance of 

 this kind among British species, nor, so far as I know, 

 among any but distinctly pelagic forms. And it is 

 remarkable that, with this extreme length of the 

 antennae, there is usually asssociated a greatly in- 

 creased development of the apical lash of swimming- 

 set^ with which the organ is armed. Sometimes also, 

 as, notably, in the pelagic genus Euchceta, the mar- 

 ginal hairs, though few, become wonderfully long. The 

 antennas have, however, other important functions 

 besides those of locomotion ; some of the variously 

 formed setae with which they are in most cases largely 

 provided act, no doubt, as organs of special sensation 

 the more simple hairs, perhaps, as tactile, the flattened 

 and club-shaped setae as olfactory organs. The flat- 

 tened, ensiform kind of appendage is seen on the 

 antennae of most of the Harpacticidce (e. g. Idyafurcata, 

 Plate LXVII, fig. 2 a), and the club-shaped form is seen 

 in great force in Isias clavipes (Plate VII, figs. 4, 5) as 



