XXIV 



THE VOYAGE OF H.M.S. CHALLENGER. 



are long and slender, the brush of cilia is much more extensive and is carried nearer 

 the distal extremity than in Palinurus. In this genus the lateral spines (Fig. VII., h") are 

 smooth on each side and curl over to meet one another and protect the membranous cilia 

 that lie between. In this genus these organs (Fig. VII., b'") have parallel sides and ter- 

 minate in a rounded extremity, the apex of which, as M. Robin says, carries a hyaline body. 

 Mr. G. L. Gulland 1 traces out what he considers the genealogy of these hairs in the 

 Crustacea, starting with a primitive seta, allied to a fringing seta, but not so flattened. 



This ideal setae stood over a wide canal ; the 

 lumen was closed, there was a single row of 

 bristles on each side, and a nerve-ending 

 attached to its base. Now these fringing 

 setse originated in one direction, and the 

 sensory setae originate in another ; these were 

 at first primary tactile setae, which became 

 modified in three directions, to give rise to 

 auditory, olfactory, and tactile setae. He 

 does not in his paper discuss in detail the 

 structure of the olfactory and auditory setae, 

 but restricts his observations to the considera- 

 tion of the tactile and fringing setae. 



He furthermore remarks 2 that in addition 

 to the sensory hairs " there is a ring of tactile 

 setae set rather far apart round the distal 

 margin of each segment, the points of which 

 are directed forwards ; they are of the usual 

 type, but very small, often not exceeding 

 - l mm. in length on the two or three most 

 distal segments where the olfactory setae are 

 absent, the tactile setae are longer and more 

 numerous on the last segment." " 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 (outer 

 flagellum), 3 and one or two isolated setae near it ; on the inner margin is a row of fringed 

 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 in 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 



Fro. VII. — Panulirus. Antenna — b\ outer flagellum ; 

 6", section of same ; b'", sensory cilium. 



1 Proc. Roy. Phys. Soc. Edin., vol. ix. p. 159, 1885-86. e Loc. cit., p. 160. 



:i This cannot be homologous with the exopodite since it springs from the third joint, whereas the exopodite 

 (basecphysis) springs from the second or basisal joint. 



