ON OUR PRESENT KNOWLEDGE 01' THE CRUSTACEA. 43 



right angles, and is directed upwards towards the organ, and ends in a 

 point ; near its base this long plate is pierced by a great oval aperture, and 

 over this opening is stretched a thin elastic membrane, which we call the 

 internal auditory membrane, and near which the auditory nerve appears 

 to terminate ; some small bundles of muscles arc attached to the extremity 

 of the osseous plate, which supports also the opercular disk of the auditory 

 tuberelo, and which by its form recalls somewhat the stirrup (bone) of the 

 human ear : moreover on the anterior border of the exterior opening, which 

 is shut by the dish, there raises also a small plate, which is parallel to the 

 internal auditory membrane ; and when the anterior muscle of the little 

 operculum is contracted so as to turn slightly this little apparatus forwards, 

 the delicate membrane to which wo allude becomes more and more stretched. 

 After the researches of M. Savart on the transmission of sound, we know 

 that an aperture closed by a thin and delicate membrane is one of the circum- 

 stances most favourable for increasing the power of an acoustic organ 



It therefore may ho assumed that this kind of tambourine that we have 

 described as covering the external ear of the Crayfish serves to communicate 

 to the auditory nerve the sensations that are transmitted to it, and affect 

 them in the same manner as if the nerve was in direct communication with 

 the external membrane. The mechanism by means of which the auditive 

 membrane is alternately extended and relaxed is analogous to that which 

 is produced in the human car by that osseous chain which traverses the 

 tympanum, and its effects should be the same. It must serve to augment or 

 diminish the extent of the undulations on the vibrating membrane, and to 

 moderate the intensity of the sounds which strike the ear. 



u The existence of the long rigid lash belonging to the second pair of 

 antenme, which is in connexion with the auditory organ, appears to be 

 another circumstance of a character that must facilitate the perception of 

 sound. This opinion has already been enunciated by M. Strauss, and appears 

 to agree well with the results of M. Savart." 



In most of the Brachyura the entrance to the organ in this pair of antennae 

 is covered and protectee! by a movable operculum, and again covered by the 

 several appendages of the mouth, a situation of much value in enabling the 

 animal to appreciate the character of food it is about to consume, but one 

 that canuot be available for an acoustic organ, seeing that sound could not 

 reach it unless interfered with to an important degree. I therefore fully 

 indorse the opinion, which I think all recent observations demonstrate 

 as true, that the second pair of antenna) is the scat of the olfactory nerves, 

 while the first contains the auditory apparatus. These antenna; arc frequently 

 developed with great power; and in the genera OoropMwn and Podocerus 

 they arc frequently used for climbing, and not improbably for clasping 

 foe and friend. 



The fourth pair of appendages is represented by the mandibles (the 

 protognaihe of M. Milne-Edwards's nomenclature of 1854, 'Annales des 

 Sciences Naturelles'). The correlation that this pair of limbs undergoes is 

 not very extreme. 



We sec them in the simplest form most probably in N&alia, where they 

 differ little from a truncated pair of appendages, the molar or masticating 

 surface being represented by a pair of opposing tubercles attached to the first 



joint. 



In the Brachyura the tubercle is increased to a maximum, while the rest 

 of the appendage is reduced to a minimum. This exists to a greater or less 

 degree throughout the several orders of Malacostraca, varying in shape and 

 altering in form, but universally present with the same functional powers. 



