MUSEUM OF COMPARATIVE ZOOLOGY. hex 
plates being perpendicular (!) to the membrane. In Fig. 30 the “ kry- 
stalloide Plattchen” of the tapetum appear as irregular, angular, more or | 
less lozenge-shaped bodies, composed of a granular central mass and a 
broad rim of uniform thickness, in the substance of which is located the 
pigment which gives the “‘ Plattchen”’ their peculiar color. _ 
Graber has apparently fallen into an error both as regards the location 
and the direction of the elements which compose the tapetal layer. It is 
not likely that the tapetum in Tegenaria differs so fundamentally from 
that of Agelena. It is probable that Graber has mistaken the posterior 
ends of the retinal cells for the corresponding ends of the so-called 
vitreous cells.* 
Grenacher (’79, p. 55) omitted a consideration of the tapetum for two 
reasons, — because (1) it presents nothing of importance for the compre- 
hension of the simple eyes and their relation to the compound eyes; and 
(2) the method of examination would of necessity have been different, 
since the employment of nitric acid to depigment the eye destroys the 
tapetum in a very short time, without leaving a trace of it. 
Without entering into a discussion of the nature of the tapetum, or its 
prevalence in the eyes of spiders, I wish to call attention to a few facts 
which appear to me of deep interest, and possibly of fundamental impor- 
tance, in any attempt to appreciate the morphological bearings and the 
functional capabilities of such eyes. ; 
No one, I believe, has hitherto called attention to the distribution of 
tapeti further than to indicate, as Leydig has done, that certain spiders 
do, and others do not, possess this structure. My examinations have not 
been sufficiently numerous to allow a very trustworthy conclusion to be 
drawn from them; but I have been impressed by the fact that, in the 
few cases examined, the tapetum, when present, was limited to the lat- 
eral anterior and to the posterior eyes; that the anterior median pair 
does not possess such a layer. When it is remembered that a division 
of the eyes into two groups is necessitated by the different types of 
bacillar development,t and that, so far as at present observed, the groups 
* Postscript. — An examination of sections of the posterior median eyes (Scheitelau- 
gen) of Tegenaria domestica, which Mr. Parker made at my suggestion soon after his 
return to Cambridge in August, has confirmed my opinion that this species does not 
differ essentially from Agelena in the position of the tapetum. It is certain that 
it lies beneath the retinal layer, and is in no sense adjacent to the pre-retinal 
membrane. . 
+ For convenience of reference I shall call the group embracing only the anterior 
median pair in Agelena the group with pre-nuclear bacilli, or, briefly, pre-nuclear 
group (Graber’s post-bacillar); the remaining six eyes in Agelena will then consti- 
