KONGL. SV. VET. AKADEMIENS HANDLINGAR. KAND 



34. n:o 8. 



Since Grenacheb and Exner and others have published their excellenl worka <>n the eyeo "I 

 the Arthropoda, there can 1»' no foundatiou for speaking of the resemblance of the trilobitic 

 eye with that of Limulus, aa this genus stånds completelj isolated amongsl all Arthropoda 

 in thnt respect. There is, as stated above, a certain resemblance between the cornea <>| 

 Peltura and thal of Limulus, bu1 this is not yei ripe for a discussion. Nor is there any 

 evidence for correlating the eyes of the trilobites with the eyes of the Phyllopoda. Beb 

 närd thinks that the s<> called eye of the Paradoxidse has beeu formed upon the same 

 plan as that of Apus. There is nötning to prove this hypothesis thal the facial ridge 

 or any part of it ever had been a visual organ, and the evidence al hand rather tends 

 in n contrary direction. 



There are signs of long physiological and anatomical efforts to prepare the deve- 

 lo|)inent of the eyes on the free cheek, as revealed through the long series of blind tri- 

 lobites. A system of radiating blood vessels, similar to those described above as covering 

 the inside of the head in some older genera, all issuing from the scallop in the free 

 cheek, where låter the eye had to lind its place, have Left their stamp, their mark on 

 the surface of the free cheek. They attest the great vital activity which was so intense 

 at the point were the cm- was to be formed. We give the figures of two such cheeks 

 of different types. One from Parabolina spinulosa (pl. V fig. 31) is the niorc common, 

 where six or more isolated trunks radiate from the semilunar ridge round the indenture 

 and subdivide in branchlets which cease near the lateral margin of the cheek. It may 

 be that it is an annular vessel near the indenture that feeds them all and that this pro- 

 bably is in connection with the great central circulatory system. In Olenus (pl. V fig. 

 29) the vessels are partly anastomosing and form a reticulate system and they are studded 

 with minute wartlets. Another sign, which may lie taken as ;i preparation, is the ele- 

 vated rim around the scallop, which is so prominent in several of the Cambrian genera, 

 but which does not embrace any facet bearing cornea. 



From what has bcen stated above the following conclusions have been arrived at. 



1. The plurality of the genera living during the Cambrian period were blind and 

 it was first at the close of that period, in the Olenus schists, that genera with real visual 

 organs appeared. There may have heen oculate trilobites earlier, as Solenopleura, hut we 

 know nothing of their eyes. 



2. The primordial glabellar pleuron which was metamorphosed into a facial ridge 

 is no visual organ. It is in the Olenidie nothing hut the elevated line made in the test 

 by the subjacent main trunk of the circulatory system. It swells out in a node, »palpe- 

 bral lobe», but not before the facial suture has been formed. In the genera where there 

 is no facial suture, there is no node. In the Paradoxidas where the ridge i> of a diffe- 

 rent origin, there is no node, though there is a suture. 



The four types of eyes in the trilobites have probably succeeded one another in 

 the following chronological order: 



1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic; 4) aggregate. 

 The oldest known representatives for each type are for 1) Harpides rugosus in the Cera- 

 topyge limestone of the Lower Silur., for 2) Eurycare, in the Cambrian Olenid schists, 

 division 2, for 3) Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for 4) 



