PALEOZOIC ARTHROPODS 279 



RELATION OF THE PALEOZOIC ARTHR0P0D8 TO THE STRAND-LINE^ 

 BY JOHN M. CLARKE 



None of the Crustacea have shown a wider distribution than the trilo- 

 bites. As to these trilobites there seems to be little in their mode of life 

 to throw light on oscillations of the strand. They were not very sensitive 

 organisms. Their primitive composition accounts for that in no small 

 measure, so they adapted themselves to bathymetric differences of some 

 considerable degree. We find their moults in the sands and the shales 

 and the limestones of the Paleozoic, and doubtless some part of these 

 have been washed out of their proper depth into the debris of the bottom, 

 but their jointed skins are found as well in all these deposits, and we 

 know they lived in shallow sands and deeper muds within perfectly easy 

 reach of land waste. Their ready adaptability and their locomotive 

 powers carried them easily over differences in sediment and depth, so in 

 the habitude of these creatures there is really little to serve as a guide to 

 geographic changes. In their anatomy there is more. I have long been 

 impressed with the possibility that the relative development of the eye in 

 these and other crustaceans, whether simple or compound, might afford 

 some clue to the bathymetric conditions governing these animals. With 

 our old knowledge it has seemed that the highly developed compound 

 lenses in- all the later trilobites, eurypterids, and in all the early shrimps 

 and crabs must be a definite response to the amount of light received in 

 their habitats; that the vaguely developed visual area in many of the 

 Cambric trilobites, sensitive to light only in a general way along streaks 

 on the head, should express a depth of water corresponding to a mini- 

 mum of light received, but there were incongruities in these conceptions 

 long before we knew the reasons for them, ^glina, with its enormous 

 compound eyes, among its lensless associates of the Cambric, the unde- 

 niable evidence of shallow-water condition, in which the unspecialized 

 eyes of the Cambric trilobites were produced, the counter-evidence of 

 the highly developed ommatidia in the Phacopes and Dalmanites of the 

 deeper waters of Siluric and Devonic, Walcott's demonstration that the 

 visual areas in Olenellus bear lenses — these and similar incongruous con- 

 ditions in the present deep sea from the Cystosoma, with its tremendous 

 ocular development to the blind crab Willemoesia, go to show that the de- 



1 Manuscript received by the Secretary of the Society May 27, 1911. 



