72 BULLETIN OF THE 



the further modifying statement that it may be double, whereas the oth- 

 ers are the result of the activity of only a single layer of cells. Grabers' 

 conception of the " Zwischenlamelle " — as a direct prolongation of the 

 iutegumental " Binnen-Cuticula," from which the sclera proper branches 

 off on the inner or deep side towards the nervus opticus — is to be so far 

 amended as to make both sclera and cuticula to branch from the " Zwi- 

 schenlamelle," rather than the sclera and " Zwischenlamelle " to branch 

 from the cuticula. Either conception is to that extent faulty that there 

 is no such thing as a branching off or a splitting, but quite the contrary, 

 — a fusion. It is possible that some of the lines seen by Graber within 

 the " Zwischenlamelle " (and explained by him as the result of the ordi- 

 nary stratification of cuticular membranes) are indications of the plane 

 along which the fusion between the component layers of this pre-retinal 

 membrane took place. 



The brilliancy of the eyes of many spiders, to which Duges ('36, 

 p. 177) was the first to call attention, was investigated by Leydig, but it 

 has received little or no attention from recent writers. 



Leydig ('55, p. 439 ; '57, p. 254 ; '64, p. 48) describes the structure 

 which is the cause of this brilliancy as a tapetum, M'hich is either con- 

 tinuous, lining completely the fundus of the eye, or, in some species 

 (Clubonia claustraria, Hahn, and Theridium sp. ?), interrupted by a band 

 of black pigment which traverses its middle in wavy lines.* 



In Phalangium the tapetum is not continuous, but consists of isolated 

 scales ("Flitterchen"). In still other cases (Lycosa saccata, and several 

 species of Epeira) it forms a narrow band at the anterior rim of the eye- 

 pigment, but becomes visible (as radial streaks lodged in the dark pigment) 

 only after the eyes are dissected out. The tapetum usually consists of 

 scales of the same kind as those which are met with in the tapetum of the 

 fish's eye. They are minute, iridescent plates, which lie close together, 

 and are separable only when subjected to strong pressure. In other 

 cases (Phalangium, Micryphantes) the tapetum is composed of spherules 

 larger than the pigment-granules. 



Graber's ('79) account of the tapetum in Tegenaria domestica 

 (" Scheitelauge") is limited to the description of his Figs. 27 and 30. 

 In the former, the " bliiulich griin schimmerndes Tapetum" is repre- 

 sented as composed of numerous minute plates, forming a stratum on 

 both sides of the pre-retinal memhrane (!), the long axes of most of the 



* In Clubonia claustraria this Mack wavy line corresponds, according to Leydig, 

 with the major axis of the oval eye. 



