MUSEUM OF COMPARATIVE ZOOLOGY. 55 
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 difference 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- 
ston 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 ; 7. 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 ‘ Retinaschlaiuche.” 
His “ parietal pigment- and matrix-zone of the retina” was not reviewed 
by Grenacher, but is considered at some length by his Jater 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 
hitherto 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. 
