No. I.] EYES OF ARTHROPODS. 219 



Neophalax, or Corethra have been examined by means of sec- 

 tions. The superficial resemblance of the organs, however, 

 both in position and structure, with the dorsal extension of 

 Acilius is such that we may confidently expect sections to show 

 a similar agreement in their minute anatomy. 



There is another set of organs that at one time I felt inclined 

 to include under this head ; they are the organs described by 

 Weismann, Leydig, and Claus, as ih^ frontal Smnesorgane of 

 the Phyllopod Crustacea. The double cells and rod-like in- 

 closures of these organs, as described by Claus, are suggestive 

 of the double rods and cells in the dorsal extension oi Acilius 

 and Dystiais. The fact that the nerve supplying these organs, 

 in some Daphniden, is connected with the three-fold ocellus is 

 probably of no significance and of secondary importance, as 

 Claus states ; for the undoubtedly homologous organs, in 

 SimocephaluSy and in Branchipus, are supplied by independent 

 nerves arising directly from the brain. The chief difficulty in 

 comparing these organs with those in Acilius is their wide 

 separation from the ocelli from which they are supposed to 

 originate, and the fact that the insertion of the nerves supply- 

 ing them is some distance from that of the nerve supplying the 

 eye to which these peculiar organs presumably belong. This 

 separation, however, may be secondary, and perhaps more ap- 

 parent than real, as is the case with Acilius in its later larval 

 stages. In an adult Limenctis, Grobben (2) has placed the 

 frontal Sinnesorgan just in front of and close to the compound 

 eyes, a condition that corresponds very nearly with that seen 

 in larval Insects. At present, however, it seems to me that we 

 can only indicate some points of resemblance between the 

 frontal organ of Crustacea and the dorsal extension of the 

 ocellus of Acilius. It is very possible that embryological studies 

 directed especially to this point may furnish us with evidence of 

 a closer relation between these problematical organs in Phyllo- 

 pods and the dorsal extension of the ocelli in Insects. 



If the compound Arthropod eye is in reality formed of two 

 parts, at one period in the development supplied with separate 

 nerves, but having a common optic ganglion, then it is possible 

 that we may find, and indeed we should expect to find, among 

 all the variations in structure of the Arthopod eye, that in some 

 species the two parts had retained their embryonic character, 



