Botany and Zoology, 109 



von Mold, was one o\' the first among German botanists who in- 

 troduced into the study thai strict method of thought which had 

 long prevailed in physics, chemistry, and astronomy; but the re- 

 searches of the last ten or twelve 1 years have unfortunately shown 



that Nageli's method has been applied to facts which, as tacts, 

 were inaccurately observed. Darwin collected innumerable facts 

 from the literature in support of an idea, Nageli applied his strict 

 logic to observations which were in part untrustworthy. The 



irices which each of these men rendered to the science are still 

 acknowledged; but my estimate of their importance for its ad- 

 vance would differ materially at the present moment from that 

 contained in my History of Botany. At the same time I rejoice 

 in being able to say that 1 may sometimes have overrated the 

 merits of distinguished men, but have never knowingly under- 

 estimated them." g. l. g. 



4. On the Compound eyes of the Arthropoda ; by J. M. 

 (i vkkk. (Communication dated Albany, March 1st, 1890.) — 

 The recent (February, 1890), number of Studies from the Biolog- 



' Laboratory of the Johns Hopkins University contains a very 

 interesting paper by Dr. S. Watase on the morphology of the 

 compound eyes in the Arthropoda. The developmental and ma- 

 ture characters of the optical unit or ommatidium are shown for 

 Serolis, Talorchestia, Cambarus, Callinectes and Limulus (lateral 

 eyes). In the last named, the ommatidia are covered by a sim- 

 ple layer of integumentary chitin, the cornea, which is thickened 

 over the opening of each eyelet, forming a simple dioptric organ 

 or lens. Thus the inner surface of the cornea of Limulus is cov- 

 ered by a multitude of minute elevations, corresponding numeri- 

 cally to the ommatidial cavities. In Serolis, representing the 

 Isopod type of compound eye, a discrete portion of the chitin is 

 enclosed within the ommatidial cavity forming a subspherical 

 crystalline cone. Dr. Watase suggests in a foot-note that the 

 structure of the eye in Phacops, which I endeavored to elaborate 

 in the Journal of Morphology, vol. ii, No. 2, appears to corres- 

 pond more closely with that of Serolis and the Isopods than with 

 that of Limulus. After reading this admirable paper I have no 

 longer any doubt that the type of trilobite-eye which was termed 

 by me holoc/irool (exemplified by Asaphus, Proetus, and in 

 fact nearly all trilobites except the Phacopida? and Harpidas), is 

 essentially identical with that of Limulus ; but I am not prepared 

 to concede the similarity between the schizochroal eye (Phacops) 

 and that of Serolis. It would simplify the correlation of what is 

 known in regard to the schizochroal trilobite-eye if we were enti- 

 tled to assume, as does Dr. Watase and as was done by Quen- 

 stedt and Burmei9ter, that a chitinous envelope originally 

 covered the entire visual surface but has invariably been lost ; 

 this, however, I do not believe is any longer a legimate assump- 

 tion. Xo trace or evidence of such a continuous corneal envel- 

 ope has ever been seen, though these fossils have been under the 

 most careful scrutiny for over fifty years. I have myself exam- 



