CRUSTACEA. 



769 



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 



Fig. 398. 



Auditory apparatus of the Maja in its natural position, 

 showed by removing the carapace and the vis- 



cera. 



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. Savart 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 antennae of the second pair, and its 

 immediate communication with the organ of 

 hearing cannot, it might have been presumed 

 a priori, be unimportant as regards the per- 

 ception of sound ; and this is found to be the 

 case in fact;f for from the beautiful experiments 

 of M. Savart we learn that the addition of a 

 rigid stem 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 



Rechcrches sur les usages de la membrane du 

 tympan et de 1'oreille externe, Journal de Physio- 

 logic de Magendie, t. iv. 



t Strauss - Drucktieim, Considerations generates 

 sur I'anatonrie des Crustaces, p. 419. 



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 insects are simple or compound ; 

 but this division is inadequate to give us any 

 proper idea of the various forms under which 

 these organs present them selves 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 eyes. 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 tlje 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 Cyamae, and 

 the Apus. The number of these simple eyes 

 never exceeds two or three. 



A step in the complexity of the organ of 

 sight is presented to us in the eyes of 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 



' On the structure of the eyes, vide Swammer- 

 dam, in the Collection Academique, partieetrangere, 

 t. v. p. 170. Cavolini, Memoria sulla generazione 

 dei Pesci et Dei Granchi. Strauss, op. cit. 

 J. Mutter, Zur vergleichenden Physiologic des 

 Gesichtsinnes etc. Ann. des Sciences Naturelles, 

 t. 17. Milne Edwards, Hist. Nat. des Crustaces, 

 t. i. p. 114. 



