SE1STS0EY OEGANS OF ARTHROPOD A. 261 



In addition to these tactile rods special organs resembling them 

 are found on the antennas of the Crustacea and Insecta ; they are 

 sometimes of considerable size, and are innervated in the same way 

 as are the tactile rods. In the Crustacea they are formed only on 

 the inner (anterior) pair of antennas. In the Insecta they are much 

 shorter, and are conical in form. Their position, in addition to the 

 circumstance that they are less long than the indifferent bristles, or 

 are placed in depressions, makes it probable that these organs have 

 another function, and it is very easy to suppose that they arc organs 

 of smell, or at least of a sensation very much like it. In this case 

 the antennas, by the differentiation of special nerve-endings, have 

 more than one function, and do not merely preside over the sense of 

 touch. 



Auditory Organs. 

 § 202. 



Auditory organs are not widely known in the Arthropoda, no 

 sign of them having been seen in the Myriapoda and Arachnida ; 

 on the other hand, in some divisions of the Crustacea and Insecta, 

 organs may be made out, which appear to be adapted for the sensation 

 of sound. 



There are two principal forms of the organ, which arc exactly 

 correlated with the medium in which the animal lives. One form is 

 found in the Crustacea, and consists of a saccular space, formed by 

 an inpushing of the integument ; it sometimes remains open and is 

 sometimes closed. These auditory vesicles lie, in most of the 

 higher Crustacea, in the basal joint of the internal antennas. Thus 

 in Leucifer, Sergestes, and other Malacostraca, as also in the 

 Arthrostraca (Hyperida), a pair of these organs may be found in front 

 of the cerebrum. As secondary structures they may also be found 

 on other parts of the body. Thus in the Mysidas, they lie in the two 

 inner lamellas of the fan of the tail. There are firm structures, 

 otoliths, in the auditory vesicles, which, when the vesicles are closed 

 (in Mysis and Hippolyta), consist of a concretion, which is held fast 

 by fine, regularly-arranged hairs. When they are open, as they 

 are very commonly among the Decapoda, and also in Tanais, the 

 orifice is greatly complicated. The place of the otoliths is here taken 

 by grains of sand brought in from the exterior ; these are regularly 

 attached by special hairs, which arise from the wall of the auditory 

 vesicle. They are like the other hairs of the integument, but are 

 distinguished from them by not having their shaft directly con- 

 nected with the floor of the vesicle, most of them standing on a fine 

 membranous process, to which endings of nerves pass. In this they 

 agree with the rod-like processes which carry the otoliths in the 

 Mysidas, to which nerves likewise pass. The auditory nerve in these 

 forms, in which the auditory vesicle is embedded in the internal 



