Dr. A. Farre on the Organ of Hearing in Crustacea. 383 



rate manner, however the current employed may vary in its energy. 

 3. An instrument for ascertaining readily what degree of the gal- 

 vanometric scale corresponds to half the intensity indicated by any 

 other given degree. 4. A means of employing the same delicate 

 galvanometer to measure currents of every degree of energy, and 

 in all kinds of circuits. 5. Processes to determine the deviations of 

 the needle of a galvanometer corresponding to the degrees of force, 

 and the converse. 



4. * On the Organ of Hearing in Crustacea." By Arthur Farre, 

 M.D., F.R.S. 



The author finds that in the Lobster (Astacus marinus), the organ 

 of hearing consists of a transparent and delicate vestibular sac, which 

 is contained in the base, or first joint of the small antennae ; its situa- 

 tion being indicated externally by a slight dilatation of the joint at this 

 part, and also by the presence of a membrane covering an oval aper- 

 ture, which is the fenestra ovalis. The inner surface of the sac gives 

 origin to a number of hollow processes, which are covered with 

 minute hairs and filled with granular matter, apparently nervous. 

 A delicate plexus of nerves, formed by the acoustic nerve, which is 

 a separate branch supplied from the supra-cesophageal ganglion, is 

 distributed over the base of these processes and around the sac. 

 Within the sac there are always found a number of particles of sili- 

 ceous sand, which are admitted, together with a portion of the sur- 

 rounding water, through a valvular orifice at the mouth of the sac, 

 being there placed apparently for the express purpose of regulating 

 the size of the grains. The author considers these siliceous parti- 

 cles as performing the office of otolites, in the same way as the 

 stones taken into the stomachs of granivorous birds supply the office 

 of gastric teeth. Several modifications of this structure exhibited 

 in the organs of hearing of the Astacus fluviatilis, Pagurus streb- 

 lonyx, and Palinurus qvadricomis are next described, and an ex- 

 planation attempted of the uses of the several parts and their sub- 

 serviency to the purposes of that sense. 



The author concludes by a description of another organ situated 

 at the base of the large antennae, which it appears has been con- 

 founded with the former by some anatomists, but which the author 

 conjectures may possibly constitute an organ of smell. The paper 

 is accompanied by illustrative drawings. 



5. " A statement of Experiments showing that Carbon and Ni- 

 trogen are compound bodies, and are made by Plants during their 

 growth." By Robert Rigg, Esq., F.R.S. 



The author, finding that sprigs of succulent plants, such as mint, 

 placed in a bottle containing perfectly pure water, and having no 

 communication with the atmosphere except through the medium of 

 water, or mercury and water, in a few weeks grow to more than 

 .louble their size, with a proportionate increase of weight of all the 

 chemical elements which enter into their composition, is thence 

 disposed to infer that all plants make carbon and nitrogen ; and that 

 the quantity made by any plant varies with the circumstances in 

 which it is placed. 



