INTRODUCTION. XXXi 



is membranous. Within the cavity and immediately be- 

 hind the little opening before mentioned, is a minute 

 vesicle filled with fluid, which conveys the vibrations to 

 a branch of the antennal nerve, which is expanded 

 upon the vesicle. This is the simple apparatus ; but 

 it is sufficient to receive and convey to the sensorium 

 the imperfect sonorous vibrations to which they are 

 subject. 



The visual organ is essentially similar to that of in- 

 sects. The eyes are compound in all the higher forms, 

 and those of the Edriophthalma do not differ essentially 

 from those of the Podophthalma, excepting in the ab- 

 sence of those movable peduncles by which the eyes of 

 the latter are distinguished. The optic nerve, the lenses, 

 the facets of the cornea, the pigment, are alike in all, and 

 in all resemble generally the same organs in insects. 

 There is one peculiarity, however, which is found in 

 certain species which live in such places as are inacces- 

 sible to light, or to such degrees of it as would render 

 eyes in any way useful. In Calocaris, for instance, a 

 little prawn-like animal, inhabiting very deep water, and 

 ordinarily immersed in mud, the eyes and their peduncles 

 do not differ in form from those of the other Palaemo- 

 pidae; but the vision is wanting. There is no pigment, 

 there are no corneal facets j the organ is evidently rudi- 

 mentary and merely formal. Mr. Westwood has recently 

 made known through the Linnean Society a form of 

 Edriophthalma, inhabiting a deep well, a species in which 

 there is no external appearance of eyes whatever ; Mr. 

 Newport has, however, by his accustomed accuracy of 

 dissection, shewn that in this case also, a rudimentary 

 visual organ exists underneath the cephalic crust. 



