Mr G-idland on the Sense of Touch in Astacus. 177 



fringing seta was developed by the loss of the nerve-filament 

 ill situations where increase of surface, straining, or other 

 mechanical function was necessary, by the seta becoming 

 flatter and longer, by a slight complication of the articulation, 

 and, at a comparatively late stage and in consequence of the 

 loss of the nerve, by the closure of the lumen. 



From the primitive form, in another direction, were developed 

 the various sensory setae ; all the setee which remained sensory 

 were probably primarily tactile, since the sense of touch is 

 undoubtedly the most ancient and most v/idely diffused of the 

 senses, and from this form the secondary tactile, auditory, and 

 olfactory setae were evolved. The auditory setse retained the 

 side bristles/ probably because these were useful in collecting 

 minute vibrations in the surrounding fluid, but developed a 

 complicated mode of articulation (Hensen, loc. cit), and were 

 (in Astacus) confined to a closed sac on the antennules, where 

 their extremities were imbedded in the otoliths. 



In another direction diverged a form from which the 

 olfactory and tactile setae were developed by another process 

 of separation. The characteristics of this intermediate form 

 were the mode of articulation, which differed only in the 

 amount of movement allowed from that of the fringing seta — 

 and was thus much simpler than that of the auditory seta — 

 and the entire, or almost entire, loss of the secondary bristles. 

 From this, again, the olfactory setae were developed in one 

 special and sheltered position, the antennule. They retained 

 the flat shape, and in most forms have entirely lost the 

 secondary bristles, but in Thysanopoda (PI. YIIL, Fig. 6, a, h) 

 a few of these have been retained on one side, to testify to 

 the oricfin of the setae. On the other hand, the tactile setae 

 were developed wherever the animal would have to come in 

 contact with rougher impressions, and therefore appeared 

 mainly at the extremities of the appendages, so as to give the 

 animal as long a warning as possible of the nearness of 

 danger or difficulty. These setae have completely lost the 

 secondary bristles, though a reversion to the ancestral type, 

 or survival of it, is to be seen in the occasional slight serra- 

 tion of the distal portion in both Astacus and Thysanopoda. 



1 Of. Huxley, Crayfish, p. 117 ; aud Hensen, loc. ciL, p. 348, and Fig. 38. 

 VOL. IX. M 



