KNOWLEDGE OF THE HYDRACHNIDAE. 265 



a single corneal facet with its subjacent pigment mass of such 

 arthropoda.* 



It is consequently not the supplementary optic organ that it is 

 here proposed to identify with the missing antennary element of 

 the Arachnid type. There in fact very generally lies between 

 these and what are usually accepted as the succeeding pair of 

 appendages or mandibles of the Hydrachnidae a pair of isolated 

 bristle-like hairs of such relative size and position as to present 

 in many instances the aspect of minute antennae ; these hairs 

 are associated usually with an apparently glandular enlargement, 

 and moreover, what is of yet more importance, are connected 

 by distinct filaments with the cerebral nerve-mass. This seti- 

 form structure apparently coincides in function, as it does in 

 position, with the first pair of antennae, hitnerto supposed to 

 be wanting in this group. It doubtless functions similarly as a 

 tactile organ, and with reference more especially to its independent 

 nerve-cord may be accepted as the homologue of the more 

 usual many-jointed primary antenna of typical Crustacea or 

 with the single one of an insect.t It may be observed that 

 two or even three contiguous hairs not infrequently take the 

 place of the more ordinary single antennary bristle, while in 

 none of the sixty or seventy species so far examined has this 

 apparent antennary homologue, though in some cases very in- 

 conspicuously developed, been found to be entirely wanting. 

 Glandular hairs analogous to these just referred to and having 

 probably a tactile function are in many forms found dis- 

 tributed at even distances round the periphery of the body, 

 though more particularly towards the posterior region, where 

 possibly they also represent the rudiments of undeveloped 

 appendages. 



In adchtion to the eyes and tactile bristles no other sensory 



* The correlation here supplied is yet more forcibly illustrated in the 

 case of the typical spiders, which possess for the most part as many as six 

 or eight optic elements, supplied, there is reason to believe, from a single 

 pair of nerve-cords. 



f Since arri\ing at this interpretation I find that the resemblance and 

 probable homology of this bristle-like hair of certain Hydrachnidae with 

 an antennary organ was first noticed by Duges (^Mcnioire snr les Acarines 

 1834), w^ho, moreover, cites the existence of a corresponding antenna-like 

 bristle in the genus Galeodes, but does not appear, however, to have sub- 

 stantiated the homology by tracing out the nerve-connections. 



