. Elements of Comparative Anatomy. VISUAL OBGANS OF AETHEOPODA. 263 cliitiuous layer of wliicli is always provided with a number of closely- applied groups of pore-canals instead of a tympanum. Organs of this kind have now been recognised at tlie root of the posterior wing of the Coleoptera^ as well as on the base of the halteres of the Diptera. The two forms of auditory organs in the Arthropoda are indeed very different from one another in the details of their arrangements, but there is, nevertheless, a connection, for in both cases the chitino- genous cellular layer gives rise to parts which
. Elements of Comparative Anatomy. VISUAL OBGANS OF AETHEOPODA. 263 cliitiuous layer of wliicli is always provided with a number of closely- applied groups of pore-canals instead of a tympanum. Organs of this kind have now been recognised at tlie root of the posterior wing of the Coleoptera^ as well as on the base of the halteres of the Diptera. The two forms of auditory organs in the Arthropoda are indeed very different from one another in the details of their arrangements, but there is, nevertheless, a connection, for in both cases the chitino- genous cellular layer gives rise to parts which carry the special end- organs ; in the Crustacea these are connected with processes of the integument, the auditory hairs; while in the Insecta they are con- verted into the small pencils, and are consequently differentiated in another direction; they remain within the dermal skeleton, and have no relations to the processes of it. No homology can be made out between these organs, owing to the diversity of their position, and from the fact that more complicated organs are derived from an elementary arrangement, which is distributed generally in the integument. Leydig, Arch. f. Anat. n. Pliys. 1855.—Geaber, V., Die tyinpanalcn Sinnesap- parate cTer Orthopteren. Denkschr. d. Wiener Acad. M. N. CI. Bd. XXXVI. Visual Organs. 204. In the visual organs of the Arthropoda we meet with points of resemblance to certain forms of eye found in the Vermes ; to those, namely, in which a number of end-organs of the optic nerves are placed directly beneath the integument (Sagitta, Hirudinea, etc.). But they have no close affinity to the more developed eyes of the Annelides, which are distinguished by the possession of a separate lens (§ 125). In Arthropoda, as in other forms, the integument is the spot at which the eye is differentiated; its mode of composition out of the elements of the integument will be understood by a reference to the sub- jacent diagram; although, of course, this does n
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