Dermal Sense- Organs of the Crustacea. 313 



means of assistance in addition to liis feebly developed eyes, 

 to enable him to distinguish the sexes in pools where the 

 water is often quite turbid." An auditory function on the 

 part of the organs of scent or smell is negatived by the circum- 

 stance that, on the one hand, auditory organs could hardly be 

 of special use to the Crustacea in the search for food and in 

 scenting out the other sex, and, on the other, that the mode of 

 attachment of these capilliform structures is of such a kind 

 that they could not well be set swinging and perceive sound- 

 waves. 



Into a discussion of the question of the power of hearing in 

 the Crustacea I will not enter here. As regards the higher 

 forms, the Decapods and Schizopods, we have the minute 

 investigations and careful experiments of Hensen {loc. cit.)^ 

 which prove that these higher Malacostraca at any rate possess 

 a very fine sense of hearing. Moreover, it has recently been 

 rendered very probable by the interesting experiments of 

 Delage* that the auditory organs of the Decapods and 

 Schizopods at the same time serve yet another function, in 

 providing for the orientation of the position of the body and 

 the regulation of the equilibrium. Whether and to what 

 extent the Arthrostraca and Entomostraca are able to hear, 

 that is to perceive waves of sound, is, according to our present 

 knowledge of the subject, still very uncertain. 



All the sensory hairs which we are not inclined to regard 

 as olfactory or auditory are termed simply tactile organs. To 

 this category belong certain sensory hairs of the first antennse, 

 and most of those upon the second antenna and its squame ; 

 in addition to these all the sensory hairs of the mouth-parts, 

 legs, and caudal appendages, and, finally, all the free sensory 

 setee upon the somites. Just as the form and arrangement of 

 these sensory hairs, which we call tactile organs, present the 

 greatest variety in the different families and species, while 

 not unfrequently several tactile hairs completely different in 

 shape are found close together upon a certain part of the body 

 in the same animal, so must we make a distinction between 

 the funciions of these capilliform structures, and, in addition 

 to coarser and finer tactile sensations, assume the existence of 

 a large number of the most widely different gradations, which 

 our perceptions are certainly unable to appreciate. 



Zoological Institute of the University of 

 Freiburg i. B., April 1891. 



* Delage, " Sur une fonction nouvelle des otocystes comme organes 

 d' orientation locomo trice," Archives d. zool. experim. 1887 (2) t. v. p. 1. 



Ami. & Mag. N. Hist. Ser. 6. Vol. viii. 21 



