Eyes of MoUuacs and Arthropods. 671 



same in the compound eye: the axial faces of the colorless eells secrete 

 rods which fuse to form awrhabdom«, the cry stalline cone. In some 

 of the simple eyes of Spiders, exaetly the same condition prevails, except 

 that the retinophorae are double like those of Molluscs, instead of five- 

 fold like those of Scorpions. This comparison will be made much 

 clearer by referriug to the diagrammatic fig-ures on PI. 32. 



Although there appears to be uo doubt about the homologies of the 

 compound eyes in Insects and Crustacea, the most diftìcult problem has 

 not been solved, and that is the relation of the compound eyes to the 

 stemma, and the ocelli of the Myriapods and Spiders. First let us 

 cousider the structure of the median, and lateral eyes of Myriapods and 

 Spiders. The question here is, are the lateral eyes double layered like 

 the median ones? Opinions, unfortuuately, seem to differ on this point. 

 I believe that both are double layered, and hence morphologically 

 identical orgaus. If it can be proved in a single instance that, in the 

 lateral eyes of Spiders, or in any of the ocelli of the Myriapods, such 

 aB LifJiobius oi'Julus, a vitreous layer is present, giving rise to the 

 cuticular leus, it is highly probable in the absence of positive evidence 

 to the contrary, that it is always present. We need go no farther than 

 Grenachee's (59) own paper to find this evidence ; any one must be ho- 

 pelessly prejudiced not to see that, in PI. 20, fìgs. 2, 3, 4, 5 and 9 of bis 

 paper, a nucleated layer must exist between the retina and the lens. 

 But if the corneal lens is formed by a distinct hypodermic layer in one 

 instance , how are we to explain the formation of a lens in the very 

 same, or nearly related, animals without the hypodermis? The simplest 

 and only advisable course is to suppose that it likewise exists in the 

 other forms, but has been overlooked. That such errors of Observation 

 are to be expected , will be evident by recalling the conspicuous Cle- 

 ments, i. e. the bacilli and the corneal hypodermis, which bave 

 escaped notice in the compound Arthropod eye, where the difficulties of 

 Observation are far less than in the simple ones. Moreover we bave the 

 asfeiirauce of Graber that ali ocelli possess a vitreous layer. In spite 

 of the torrents of abuse that Grenacher has showered upon Graber 

 forhisdiscoveryandmisinterpretation of afew extra nuclei in the 

 retina of Myriapods, Spiders and Scorpions, it is tolerably certain, that 

 if Graber has made some unfortunate interpretations , he has also 

 made several accurate and valuable observations concerning structures 

 which bave entirely escaped Grexacher. A person who admits that 

 bis observations are full of gaps and errors , and who invites criticism 

 with the assurance that it will be thaukfully received, should, it 



