Mr Gidland on the Sense of Tovxh in Astacus. 161 



of which are directed forwards ; they are of the usual type, 

 but very small, often not exceeding 01 mm. in length. On 

 the two or three most distal segments, where the olfactory 

 setae are absent, the tactile setae are longer and more nume- 

 rous, and they are most numerous on the last segment. On 

 the endopodite the arrangement of the tactile setse is the 

 same, but here they are rather longer. On the third joint of 

 the main stem there is one large group of tactile setae on the 

 outer margin at the base of the exopodite, and one or two 

 isolated set£e near it ; on the inner margin is a row of 

 fringing setae, and all the setae on the first and second joints 

 are also of this kind, with the exception of a very few small 

 tactile ones on the inferior margin of the triangular first 

 joint. If the antennule be examined in situ, the significance 

 of this arrangement will be at once apparent ; for it will be 

 seen that only those parts which bear tactile setae are really 

 external, and that these only could receive tactile impressions, 

 since the other parts are covered by the eyes, the rostrum, 

 squame of the antenna, and the antennule of the opposite 

 side. The fringing setae along the margins of the surface, 

 which bears the opening of the auditory sac, as well as the 

 close set row of fringing setae which cover the opening, act, 

 doubtless, as strainers, and prevent the entrance of foreign 

 bodies to that delicate organ. 



Experimental. — Lemoine has already remarked that the 

 antennules are extremely sensitive, especially at the point, 

 and this I can fully corroborate. As Huxley {loc. cit., p. 115) 

 has pointed out, the olfactory setae are of essentially the 

 same structure as the tactile setae ; Jourdain's denial of this 

 resemblance Qoc. cit, pp. 405, 409) is due to an erroneous 

 conception of the structure of the olfactory setae, and ignor- 

 ance of their mode of formation. I shall have more to say 

 of the olfactory setae when discussing the genealogy of the 

 various forms of setae. 



The Antennce. — On the terminal multiarticulate filament 

 of the endopodite (Huxley's procerite), the disposition of the 

 setae, which are all tactile, is just the same as in the filaments 

 of the antennules ; they are, however, more numerous, and 

 near the base of the filament rather longer. There is a large 



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