VISUAL ORGANS OF ARTHROPODA. 



263 



chitinous 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 the 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. 



Letdig, Arch. f. Anat. u. Phys. 1855. — Graber, V., Die tympanaleu Sinuesap- 

 parate der Orthopteren. Denkschr. d. Wiener Acad. M. N. CI. Bel. 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 not represent the simplest 

 condition. The articular layer of 

 the inteo-nment forms a biconvex 

 thickening over the eye (/) ; this 

 forms a refractinsr but also a defen- 

 sive organ, and functions therefore 

 as a cornealens. The eye, which is 

 formed from the hypodermic layer 

 (It), lies behind this lens. Around it 

 the hypodermic cells elongate and change their position; they become 

 pigment cells (p). The optic cup, into which project transparent 



Fig. 134. Section through the simple 



eye of a young Dytiscus larva 



(after Grenacher). 



