240 EEPOET— 1880. 



wliicli may be compared to a tympanum, or " a la membrane de la fenetre 

 du vestibule des animaux superieurs " (PI. XII. fig. 11 and 11 bis). Behind 

 this membrane, at the base of the tnbei-cle, we find a little membranous 

 vesicle full of an aqueous fluid, which receives on the inner and upper 

 surface a nervous filament given off from the antennal branch. Moreover, 

 it is capped by a spongy mass, of which Scarpa makes no mention, which 

 appears to be well adapted for an organ of hearing, although some narrow 

 bands unite it to the organ of which we are about to speak (PI. XII. 

 fig. 9a). It is this organ which has already been considered as connected 

 with the sense of smell. In the Langouste (Palinurus), in the centre of the 

 membrane that closes the aperture of the antennal tubercle (Phymacerite) 

 is a small opening which communicates with a disc-like organ (I'organe 

 en form de galette) the object of which is doubtful, and for the most part 

 among the Brachyura is entirely replaced by a small osseous, more or less 

 movable, disc. In Maia and some other short-tailed Crustacea, the dis- 

 position of this kind of operculum is very cui-ious ; we have ascertained, 

 M. Andouin and myself, that on the anterior border there exists a tolerably 

 large osseous plate which is bent at right angles and directed upwards 

 towards the organ, and forms a disc that terminates in a point ; near its 

 base this lamellous prolongation is pierced by a great oval foramen, and 

 this kind of opening is closed by a thin elastic membrane which we shall 

 call the internal auditory membrane, and near which the auditory nerve 

 appears to terminate. Fasciculi of muscles are attached to the extremity 

 of the osseous lamella, which comes from the opercular disc of the 

 auditory tubercle (Phymacerite), and which by its form resembles the 

 stirrup of the human ear ; finally, upon the anterior border of the external 

 opening which is closed by this disc, there exists also a little osseous plate 

 which is parallel with the internal auditory membrane, and when the 

 anterior muscle of the ossicle contracts, so as to be slightly thrown back, 

 all the little apparatus before the membrane to which we allude becomes 

 more and more extended. 



' After the researches made on the transmission of sound by M. Savart, 

 we know that the existence of an opening closed by a thin elastic membrane 

 is a condition most available for the increase of the power of heai-ing 

 delicate sounds. This savant has observed that pieces of cardboard which 

 are not susceptible to vibration so as to determine the form of regular 

 figures in the fine sand placed upon the surface, are capable of so becoming 

 when they were covered with a membranous disc. It is then to be presumed 

 that the kind of drum that we now describe as that of the external auditory 

 membrane of the crayfish, serves to communicate to the auditory nerve 

 the vibrations that are transmitted to it, and which afiect but little or 

 nothing the sounding parts that are not in direct communication with 

 these membranes. The mechanism by means of which the internal 

 auditory membrane can be alternately relaxed or extended is analogous to 

 that which is produced in the human ear by the action of the chain of 

 ossicles, which traverses the cavity of the ear, and its effects may be sup- 

 posed to be of the same kind. It serves to augment or diminish the 

 undulations which strike the vibrating membrane, and to modify the 

 intensity of the sounds which strike the ear.' 



That this organ is not connected with hearing is now, I believe, 

 accepted by those who have inquired into the subject, since organs 

 resembling otolithes have been found in the coxa of the first pair of 

 antennse, and in the inner ramus of the posterior pair of pleopoda. 



