176 Proceedings of the Royal Fhysical Society. 



podite; but a few experiments convinced me that the centre 

 for this action was in the central nervous system, and pro- 

 bably in the ganglion of the cord corresponding to the great 

 claw. I found that if I divided the nerve in the appendage, 

 or destroyed the ganglion corresponding to the claw through 

 the sternal wall, while the peripheral ganglion remained 

 intact, no amount of stimulation of the tufts of setae would 

 bring about closing of the pincers. One is therefore, I think, 

 entitled to regard this ganglion as sensory in function, which 

 opinion also its position confirms, since it lies on the nerve 

 at a point proximal to the junction with it of all the sensory 

 nerves returning from the tufts of setse in the " index " of 

 the propodite. It therefore probably collects the impressions 

 from the ultimate nerve-end organs, and transmits them to 

 the central nervous system. 



The Genealogy of the Set^. 



It is, perhaps, rather premature to attempt to trace the 

 genealogical relationship of the setae, but I shall make an 

 attempt to do so from data furnished by my own researches 

 and those of others. I regard a form nearly allied in struc- 

 ture to the fringing setae as most primitive, since these 

 present less evidence than any other of the setae of differen- 

 tiation in their mode of attachment to the integument, their 

 distribution, and uses. I should say, then, that this ancestral 

 seta was not quite so much flattened as the typical fringing 

 seta; that it stood over a comparatively wide setal canal, 

 with which its lumen communicated (as I have elsewhere 

 said, I consider the closure of the lumen to be of compara- 

 tively late appearance) ; that it was articulated to the cuticle 

 by a simple short membrane (much as the fringing setae of 

 Thysanopoda are at present) ; that there was a single row of 

 bristles along each edge of the seta ; and that, possibly in 

 every case, a nerve-ending was attached to its base, so that 

 the seta was both sensory and fringing. I may mention 

 that in a large number of cirripede Nauplii which I have 

 examined, all the setae, which are usually represented as 

 simple, were furnished with secondary bristles, extremely 

 delicate, but quite distinct. From this form the typical 



