The membrane in the greater number of the Brachyura is replaced by a small moveable os seous disc, which in the Maja and some others presents a pretty broad bony plate ( 397) at its posterior edge, detaching itself at right angles and running upwards towards the glan dular organ already mentioned. Near its base this lamellar prolongation is pierced with a -large oval opening, over which there is stretched a thin and elastic niembrane which might be named the internal auditory membrane, near to which the auditory nerve appears to terminate. This small bony lamina, which is moved by minute muscular fasciculi, recals in some measure the stapes of the human ear. Under the anterior edge of the external opening of the ear which is closed by this bony disc (fig. 398 ), is seen a small lamina parallel to the- internal auditory mem brane ; and when the anterior muscle of the ossiculum contracts so as to bring, in a slight measure, the whole of this little apparatus forwards, the membrane of which mention has just been made rests upon the bony prolonga tion, and is made tense in a continually in creasing degree ; and from the experiments of M. Sa.vart we know that all increase in the tension of thin membranes lessens their dispo sition to be thrown into vibration ;* consequently in undergoing such a modification, the kind of tympanum described must serve to moderate sounds of too great intensity, in their passage to the acoustic nerve. In other respects it is evident that the mechanism described pre sents the most forcible analogies with what we observe in the human ear, and that the ossiculum auditus here stands in lieu of the chain of small bones which exists in the organ of hearing arrived at its highest point of de velopment.
The presence of the long rigid stem formed by the antennw of the second pair, and its immediate communication with the organ of hearing cannot, it might have been presumed cl priori, be unimportant as regards the per ception of sound ; and this is found to be the case in fact ;t for from the beautiful experiments of NI. Savart we learn that the addition of a ridd stein is sufficient to render certain vibra tions perceptible, which, without this kind of conductor, are altogether inappreciable.
The auditory apparatus of the Crustacea con sequently consists. essentially of a cavity full of fluid, to which a nerve adapted to perceive sonorous impulses is distributed; which 'ele mentary and essential apparatus .is assisted in its functions by certain special organs, such as elastic membranes and rigid stems? calculated by their nature to vibrate under the action of sonorous undulations.
We have still to speak of the organ of sight. With the exception of certain parasitic species, the faculty of perceiving the existence of ex ternal objects by the medium of light is pos sessed by the whole class of Crustacea and is found dependent on a particular organ of a con siderably complicated structure situated in the head, towards its anterior aspect, superiorly or on the sides. Even the exception which
has been made is merely accidental, as it were; for in the earliest periods of their existence the parasitic Crustacea also possess eyes, and it is only as an effect of the kind of meta morphosis which these animals experience that the organs of vision disappear.
The eyes in insectsiare simple or compound ; but this division is inadequate to give us any proper idea of the various forms under which these organs present themselves to our observa tion in the Crustacea, and into the study of which we shall, therefore, enter with some attention to detail:* The least complex form under which the eyes of the Crustacea occur is that which has been designated under the name of Stemmata, smooth eyes or simple ewes. The structure of these does not differ essentially from that ob served among the higher animals. We distin guish, in the first place, a transparent cornea, smooth and rounded, which is in fact nothing More than the general tegumentary mem brane modified in a particular point. The internal aspect of this cornea is in immediate contact with a crystalline lens, generally of a spherical form ; this, again, is in contact poste riorly with a gelatinous mass analogous to the vitreous humour, and this mass in its turn is in contact with the extremity of the optic nerve. A layer of pigmentum thick and of a very deep colour, envelopes the whole of these parts, lining the internal wall of the globe of the eye up to tlie point at which the cornea begins to be formed by the thinning of the tegumentary envelope become transparent. This is what we observe in a limited number of the Crustacea, among which we may mention the Limuli, the Cyamze, and the Apus. The number of these simple eyes never exceeds two or three.
A step in the complexity of the oro-an of sight is presented to us in the eyes ocif the Nebalia, Branchipus, and Daphnia. In these, behind the cornea, which externally presents no trace of divisions, a variable number of small crystalline lenses and vitreous humours are found, each included in a kind of sac or pigmentary cell, -and terminating by coming immediately into contact with the optic nerve. These eyes are obviously made up by the con junction of several stemmata under a common cornea. The Apus, besides its pair of simple eyes, presents another compound pair, behind and at some distance from these.
The Amphiho6 Prevostii and some other Edriophthalmians present the transition from the form last described to that of truly com pound eyes,liaving distinct facets. The cornea in these is formed of two transparent laminw, the external of which is smooth and without divisions, whilst the internal is divided into a variable number of hexagonal facets, each of which is a distinct cornea, superposed upon such a conical crystalline lens, as we shall have occasion immediately to describe when speaking of compound eyes properly so called, or eyes with simple j'acets.