xxii THE VOYAGE OF H.M.S. CHALLENGER. 



but more delicate in structure. These vary in number and in thickness of clusters, but, 

 as far as my experience goes, are invariably present on the upper antennae." 



More extended research has led to these membranous cilia, or rods, being regarded as 

 sensory organs, but their exact function has not yet been definitely determined. In 

 some genera they are extremely numerous and they are usually more abundant in the 

 males than in the females. By Leydig they have been regarded as having an olfactory 

 function, but M. S. Jourdain ' says that they are each covered by a delicate chitinous 

 layer and divided into a variable number of joints ; the free end has the form of a 

 truncate cone and bears a hyah'ne process, which probably has a sensory function ; 

 within the sheath is a granular substance, derived apparently from the dermal layer, 

 or chorion, and a nerve fibril has been traced to its base. These sensory rods are 

 variously distributed in the different groups, but when the flagellum is branched they 

 occur in one of the branches only. And thus they are almost invariably present in 

 the Macrura, since the first antennae are almost invariably binagellate. Among the 

 Edriophthalma, on the other hand, it is frequently uniramous ; but even here the second 

 branch is almost universally present in a rudimentary condition in the young, and the 

 structure of the membranous cilia is essentially the same as in the other orders, but their 

 arrangement shows an immense number of variations. M. Jourdain says that the first 

 pair of antennae has no special movements, and the number of rods is not great, but my 

 own experience is at variance with these assertions, for the Amphipoda always while 

 swimming carry the flagella of the first pair of antennae elevated in the water, and slowly 

 waving about as if watching for impressions, while in the Brachyura and Anomura, and 

 in those Macrura where the flagella are short, they are kept in a constant state of vibra- 

 tion. But I agree with M. Jourdain in the belief that while admitting the function of 

 these rods, or membranous cilia, to be sensory, there is nothing in their structure to 

 prove them to be specially devoted to the sense of smell. 



M. Bobin, in a memoir on the subject, 2 after reviewing M. Jourdain's observations 

 on the sensory rods, says that in all cases we find a very delicate chitinous sheath, which 

 is penetrated by an offshoot from the hypodermic layer, and which at its base is found 

 to be in relation to a branch of the antennary nerve ; the free end is truncated and 

 carries a hyaline body, which appears to be comparable to the rods found at the ends 

 of sensory organs. These may be known as the "poils a, batonnet." The hairs are 

 cylindrical in some cases, and then the chitinous cylindrical sheath is made up of a 

 number of joints ; the basal ones have thicker walls, and are shorter than those which are 

 more distal. In other cases the hairs are stipitate and then the joints are ordinarily 

 reduced to three, and the basal- one, which is of some length, is constricted in its middle. 



A detailed study shows that the former arrangement is confined to the Podoph- 



1 Comptes rendus, torn. xci. pp. 1091-3, 1S80. 



- Jowrn. Anat. et Phys., torn. xvii. pp. 402-418, 2 pis., 1881. 



