MUSEUM OF COMPARATIVE ZOOLOGY. 65 



we must infer, it seems to me, that their explanation is equivalent to say- 

 ing that the " diplostichous " condition has arisen by a gradual sinking 

 down of the retinal area, and a subsequent closing in of the adjacent 

 epiblast to constitute the outer layer of the ommateum. The funda- 

 mental diflference between such a method and that shown by Locy to 

 exist in spiders is, that, according to the former assumption, there is no 

 inversion of the retinal area, whereas in spiders there is a complete inver- 

 sion of the more superficial of the two infolded layers. 



It must be left to future observers to ascertain whether any of the 

 monomeniscous eyes of Arthropods are, as seems possible, actually formed 

 in the manner suggested by the condition in the Myriapods ; i. e., without 

 the inversion of the retinal area. Meanwhile one examines with fresh 

 interest the conditions hitherto described in order to ascertain, if may 

 be, the probable outcome of future studies. 



Next in importance to the presence of two distinct cell-layers,* the 

 presence or absence of Graber's pre-retinal membrane will be significant. 

 In all cases where there is an obvious pre-retinal membrane, and when 

 the " vitreous " is composed of a layer of cells which abut directly (per- 

 pendicularly) upon it, I believe there can be little doubt that the retina 

 has been formed by a process of inversion. Such I think is the case in 

 the eyes of all the Arachnoids hitherto carefully studied. 



The cases among Arachnoids which will at first sight present the 

 greatest obstacle to the acceptance of this view are those of the scor- 

 pions ; it is therefore to these that most attention will be given. 



Graber has given figures and descriptions of the median eyes in scor- 

 pions, which have been reviewed both by Grenacher ('80, pp. 421-425) 

 and by Lankester and Bourne ('83, pp. 191-193). Their criticisms deal 

 especially with the nuclear conditions of Graber's " Eetinaschlauche." 

 His " parietal pigment- and matrix-zone of the retina " was not reviewed 

 by Grenacher, but is considered at some length by his later critics, under 

 the head of " Intrusive pigmentary connective tissue." 



* The presence of the third or posterior layer is unquestionably of the greatest im- 

 portance as a test of an invagination with inversion ; but I believe that it may be so 

 reduced in thickness in the adult that the negative evidence of its not having been 

 hitlierto found in any particular case should not weigh too heavily in the interpre- 

 tation. I find, for example, in the case of some adults (Tegeneria, Theridium, 

 Thomisus) that the posterior layer is indicated only by the presence of very thin, 

 flattened nuclei, sometimes so densely enveloped in pigment-granules as to be 

 almost unrecognizable, but occurring at such regular intervals as to leave little doubt 

 about their real nature. 



