The Internal Anatomy of Spiders 



tapetum can be recognized as a nocturnal eye by its colour when 

 viewed externally. In order that the eye may give forth the 

 pearly lustre it must be comparatively free from pigment between 

 the tapetum and the cornea; for if this is not the case, the re- 

 flected light will be absorbed to such an extent that the eye will 

 not glisten. 



As no postbacillar eyes (anterior median) have been found 

 in which there is a tapetum, it is probable that these eyes are 

 never nocturnal. It should be noted, however, that in a few 

 cases, as in Meta for example, the anterior median eyes have the 

 pearly white lustre. A careful study of these eyes shows that 

 there is no tapetum present, the retina immediately behind the 

 optic rods being densely packed with interstitial pigment cells. 

 The pearly lustre is due, therefore, to some other cause than the 

 presence of a tapetum, and these eyes are only apparently noc- 

 turnal. 



The pearly white prebacillar eyes of several genera (Pholcus, 

 Theridion, Agelena, Clubiona, Tracbelas, Meta, and Amaurobius) 

 were studied by Mr. Shafer. It was found that in each there is 

 a funnel-form tapetum, and that there is no pigment between the 

 tapetum and the cornea. These eyes are, therefore, truly noc- 

 turnal; and the pearly lustre is doubtless due to the presence of 

 the tapetum. 



In the prebacillar eyes of Pardosa, Lycosa, and Misumena, 

 for example, a grate-form tapetum was found. These eyes 

 are, therefore, strictly speaking, nocturnal. But as in the 

 grate-form tapetum-eyes the pigment cells extend in front 

 of the retina, surrounding the optic rods, but little light can 

 be reflected out of the eye. On this account these eyes are 

 apparently diurnal. 



To sum up the results it can be said that certain diurnal 

 eyes (the anterior median eyes of Meta, for example) are appar- 

 rently nocturnal; and that many nocturnal eyes (those having a 

 grate-form tapetum) are apparently diurnal. In other words 

 the presence or absence of a pearl} - lustre is not sufficient to 

 determine whether an eye is nocturnal or diurnal. 



The eye-muscle. — Fach anterior median eye is provided with 

 a muscle extending from the body-wall to the eye-capsule. No 

 eye-muscles have been observed connected with any of the pre- 

 bacillar eyes in true spiders. 



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