MUSEUM OF COMPARATIVE ZOOLOGY. 73 



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 nothii^g 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 tlie nature of the tapetum, or its 

 prevalence in the eyes of spiders, I wish to call attention to a few facts 

 "vj'hich 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 dpmestica, 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- 



