KONGL. sv. VET. AKADEMIENS HANDLINGAR. BAND 34. \:«> 8. 27 



Eurycare. 



Pel tu ra. 



SphaBrophthalmus. 



II. Genera with aggregate eyes of biconvex lenses. 



Acaste. 

 Chasmops. 

 Dalmanites. 

 Phacops. 



III. Genera with isolated eyes, one or several stemmata at the extremity of a straight 



facial ridge. 

 Harpes. 



Harpides. 

 (?) Trinucleus in the larval state. 



Only a few authors have before now occupied themselves with the intimate structure 



of the trilobite eye. Packard gave in 1880, in the »American Naturalist» a note on the 

 structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the 

 eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches 

 of Grexacher he still »claims that the trilobite eye was organized on the same plan as 

 Limulus». This statement is altogether wrong, and as I hope to show the trilobites have 

 had eyes entirely different from that of Limulus and instead agreeing with those of the 

 Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. Clarke published 

 an account 1 on the »Structure and development of the visual area in the trilobite Pha- 

 cops råna Greex». The aggregate eye described by him are of the type forming my 

 third group. His holochroal division embraces my first and second groups and the schizo- 

 chroal my third. 



The latest contribution to the knowledge of these eyes is found in Exner's »Phy- 

 siologie der facettirten Augen von Krebsen und Insecten», 1891, where he gives good 

 figures of the lenses of Phacops fecundus, pl. Il.figs 18, 19. He says that the palpable 

 difference in the structure of these eyes and those of Limulus point to a change in the 

 function of these eyes. 



I. Compound eyes. 



1. Eyes with prisniatic, plano-coiivex lenses. 



A pellucid, smooth and glossy integument, a direct continuation of the common 

 test of the body covers the corneal lenses, quite as is the case in so many of the recent 

 crustacea. In the plurality it is, however, difficult to discern the lenses from the outside. 



1 Quart. Journ. of Morphology, vol. II p. 253. 



