MALE QENEEATIVB SYSTEM. 



411 



apposition with a plexus of the auditory nerve, which has a separate and 

 distinct origin from the supra-oasophageal ganglion. 



(1051.) The existence of this singularly- constructed apparatus is by 

 no means universal even among the Macrourous Decapods, and in the 

 Brachyura it seems to be altogether wanting. We recognize, however, 

 in its structure all the essential parts of an organ of hearing in its 

 primitive form, viz. a distinct acoustic nerve, terminating in a plexus, 

 which is expanded upon a vestibular sac. The remarkable arrange- 

 ment of ciliated processes immediately overlying this plexus, with each 

 process filled with nerve-granules, exhibits an apparatus for extending 

 the extremities of the nerves in such a manner as to render them sen- 

 sitive to the most delicate vibration of the fluid with which the sac is 

 filled. But to heighten the effect of this, the grains of sand are added, 

 thus forming adventitious otoliths, which, moving freely in the fluid 

 contents of the sac, doubtless considerably increase the vibration of that 

 fluid. 



(1052.) In the Brachyura, or Crabs, the membrane covering the ex- 

 ternal orifice of the ear is converted into a moveable calcareous lamella, 

 from which, in some genera, a furcate process is continued internally ; 

 so that the whole, when removed by maceration, has no very distant re- 

 semblance to the stapes of the .p. 2Q7 

 human ear, and, like it, seems 

 to be acted upon by muscular 

 fasciculi, so disposed as to 

 regulate the tension of the 

 vibratile membrane, and thus 

 adapt it to receive impressions 

 of variable intensity. 



(1053.) One of the first cir- 

 cumstances calculated to at- 

 tract the notice of the anato- 

 mist who turns his attention to 

 the structure of the genera- 

 tive system, both in male and 

 female Decapod Crustacea, is 

 the complete separation which 

 exists between the organs be- 

 longing to the two sides of the 

 body ; for not only are the 

 internal secreting viscera for 

 the most part perfectly di- 

 stinct from each other, but 

 even the external sexual ori- 

 fices are equally separate and unconnected. 



(1054.) Beginning with the parts observable in the male, we will 



Male generative apparatus of Astacus fluviatilis ' 

 a a, 6, testicular mass ; c c, vasa deferentia, forming 

 by their convolutions a kind of epididymus, d d;f, 

 their external orifices. 



