The Internal Anatomy of Spiders 



median) differing conditions occur in different species. In eyes 

 with a funnel-form tapetum there is no pigment in front of the 

 tapetum; but in the second type of funnel-form tapetum-eyes 

 described above, there may be interstitial pigment-cells between 

 the nuclear ends of those visual cells that bend around behind 

 the tapetum. And in both the first and the second types of 

 funnel-form tapetum-eyes there are pigment cells between the 

 nerve fibres behind the tapetum. 



In the grate-form tapetum-eyes, not only are there pigment 

 cells between the nerve fibres behind the tapetum, but they 

 extend above the tapetum as well, surrounding the optic rods. 



In the prebacillar eyes of many spiders there is found a 

 pigmented layer similar to the funnel-form tapetum in form and 

 position and, doubtless, homologous with it. This layer has 

 been termed, unhappily, a "pigmented tapetum." But as it can- 

 not reflect light, owing to its being pigmented, it cannot properly 

 be called a tapetum. 



Finally, in some cases, a prebacillar eye is divided by a thick 

 sheet of pigment into two portions, a larger median and a smaller 

 lateral. In these divided eyes the visual cells differ in form in 

 the two parts of the retina; and the form of the tapetum may differ 

 in the two parts. 



Nocturnal eyes and diurnal eyes. — In the case of many spiders 

 that live in the dark or that frequent shady places some of the 

 eyes are pearly white. It is believed that such eyes are fitted 

 for seeing in a faint light; they have been termed, therefore, 

 nocturnal eyes. On the other hand, those eyes that are dark in 

 colour are termed diurnal eyes. Considerable use is made of this 

 distinction in the classification of spiders. It is of interest, 

 therefore, to determine whether the pearly white eyes are fitted 

 for nocturnal vision or not; and if so, to determine in what 

 respect they differ from the so-called diurnal eyes.* 



Strictly speaking, any eye furnished with a tapetum, that is 

 with a reflecting structure so placed behind the retina that the 

 light entering the eye is made to traverse the retina a second time, 

 must be considered a nocturnal eye. 



It does not follow, however, that every eye provided with a 



•At my suggestion, the working out of this problem was undertaken by Mr. George D. Shafer 

 in the entomological laboratory of Cornell University. The morphological data given below as well 

 as some of that given above are drawn from a thesis by Mr. Shafer, which, at the present writing, 

 is unpublished. 



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