OF THE TRILOBITES. 19 



SECTION VII. 



Eight years have ah'cady elapsed since I particularly described the type of this form of 

 the eye (to which Midler* first directed attention), in its most important living representative, 

 the Branchipus sta(jiiaUs, and I then showed that the eye of this animal consists of four suc- 

 cessive layers of different kinds. The external layer is a smooth, homogeneous, transparent 

 cornea. Beneath it lies a facetted membrane, which, seated in a clear substance, contains 

 rather darker, firmer, circular, apertures, of equal size, and regularly distributed in such a 

 manner that every ring is surrounded by six others, at equal distances from each other. 

 The third layer of the eye consists of egg-shaped, transparent, very hard lenses, each of 

 which is situated behind one of the little window-like apertures described, resting upon the 

 surface of the latter with its flatter extremity, and raising this a little with that convex 

 surface. The fourth layer consists of an oblong, club-shaped, crystalline body, which 

 encircles with its upper thicker end the more pointed end of the egg-shaped lens, and is 

 surrounded by a delicate membrane. A continuation of this membrane also overspreads 

 the lens, and attaches itself to the thickened margin of the little aperture before each 

 lens. Behind the crystalline body there then follows the dark pigment as the principal 

 mass of the whole eye, through which the fibres of the optic nerves extend themselves to 

 the respective ocelli, resting on the basis of the crystalline bodies, as their sheaths pass into 

 the sheaths of the crystalline body, and the lenses, and through those sheaths likewise 

 attach themselves to the facetted second membrane, f This representation of the eye,;}: 

 which is perfectly applicable to. the Trilobites with a smooth cornea, shows us that the 

 loss of the external smooth cornea immediately occasions the projection of a facetted 

 cornea, § and we therefore only need assume with respect to Phacops that their cornea must 

 have been more destructible than that of the other Trilobites, in order to explain their 

 facetted character. Sufficient reasons are also in this respect furnished to us by the propor- 

 tions of organization in existing genera. The study of all those Crustacea, for instance, 

 that are furnished with a smooth cornea, and they are only found in Articulata of that 

 description, proves to us the important fact, that the number of the separate ocelh does not 

 at all depend on the size of the whole eye, since they merely become more minute as the 

 eye diminishes, their absolute number in that case sometimes actually becoming greatei'. 

 The cornea becomes thinner in proportion to the increased size of the eye, and thicker 

 as the eye is smaller; so that very large eyes with a smooth cornea possess a thin cornca,|| 

 very small ones, on the other hand, a thicker and more compact cornea. Now Phacop.'s 



* MüUer's Archiv for 18.35, pp. 529, 613. 



t Vide Table YI, Fig. 4, and its explauatiou. 



X Vide Quenstcdt in AViegmaun's Aj-cliiv für Nat. Gesch., series 1837, i, 340 ; where the structure 

 of the eye of the Trilobites with a smooth horny membrane is correctly recognized and described. 



§ In .Toll. ]Müller's description, which we ha\e before alluded to, the facetted membrane and 

 glassy substance is not mentioned. AVe need not, however, infer from this that they ai-e wanting in 

 some eyes ; they have only escaped the attention of the observer at this first investigation, and are cer- 

 tainly met with in all the Articulata Avitli the described form of eyes. 



II Compare in these respects, for instance, the genera Branchipus and Apus, or Polyphemus and 

 Daphnia. 



