154 Mttller on the Structure of the Eyes 



Leuwenhoek and Cavolini ; in Pagurus Bernhardus, by Swam- 

 merdam ; in Limulus Polyphemus, by Andre. 



In Penaeus sulcatus, Dr. Miiller describes the cornea as sub- 

 divided into quadrangular facets, and in contact posteriorly 

 with a stratum of short crystalline masses, the lateral surfaces 

 of which are coated by a greenish opaque pigment, separating 

 them from each other. The crystalline columns, or prisms, 

 are quadrangular, perfectly transparent, very short, being about 

 as long again as they are wide, and in contact posteriorly with 

 the fibrils of the optic nerve. 



In Lucanus cervus (Coleoptera), the cornea is exceedingly 

 thick, its facets being elongated like prisms. The crystalline 

 bodies are conical, the bases being almost in contact with the 

 cornea, whilst the apices are in contact with the extremities of 

 the fibrils of the optic nerve, each of which is coated externally 

 by a violet pigment. 



A similar structure with some minor variations is also to be 

 found in Orthoptera, Hemiptera, Lepidoptera, Hymenoptera, 

 Diptera, and Neuroptera. As the general result of such obser- 

 vations, Dr. Miiller describes the structure of such compound 

 eyes as follows : Behind the facets of the cornea is situated a 

 stratum of elongated transparent prisms, in close apposition to 

 each other, cylindrical or conical, and allowing the transmis- 

 sion of light in the direction of their longitudinal axis only, 

 their lateral surfaces being coated with pigment. The propor- 

 tion between their longitudinal and transverse diameters varies 

 from 10 : 1, to 2 : 1. The anterior extremity, in contact with 

 the cornea, is sometimes smooth, sometimes rounded. The 

 pigment is sometimes black, as in Dytiscus, Blatta, PhalaenaB, 

 &c. ; at others, as in Penseus, Locusta, Gryllus, &c. yellowish- 

 white, greenish, &c. though still opaque. 



In some few cases the transparent cones are wanting, though 

 their place is even here supplied by a thin transparent mem- 

 brane, subdivided like the cornea into facets ; e. y. in Vespa 

 crabro, Papilio rhamni, Libellula quadrimaculata, ./Eschna 

 grandis. In Meloe maialis, the cornea is studded posteriorly 

 with transparent projections, very convex, and almost para- 

 bolical. 



4. Compound Eyes without facets. In Monoculus apus 

 the cornea, which is continuous with the common integuments, 

 is smooth, and without facets ; on removing it, the surface of 

 the eye presents a dense aggregate of very small semicircular 

 elevations, which terminate posteriorly in pointed cones, 

 embedded in black pigment, and connected with the tuft- 



