The External Anatomy of Spiders 



is not a good one; it is discussed at length in the next chapter, 

 where the structure of nocturnal eyes is explained. 



The anterior median eyes differ from the other six eyes in a 

 remarkable manner as regards their intimate structure and mode 

 of development. These eyes have been termed the postbacillar 

 (post-bac'il-lar) eyes; and the other six eyes, the prebacillar eyes. 

 The differences in structure between these two types of eyes are 

 discussed in the next chapter. 



In many spiders the eyes are arranged in two transverse 

 rows each containing four eyes (Fig. 73); and this is regarded as 

 the normal arrangement. Special names are applied to the 

 different eyes, the names being suggested by their relative po- 

 sitions when thus arranged. The names are anterior median, 

 applied to the two intermediate eyes of the first row; posterior 

 median, the two intermediate eyes of the second row; anterior 



Qh- 



Fig. 72. 



CARAPACE 01 \ I \K \\ TULA 

 eg, cervical 

 r], radial furrows 



FiR. 73. MODELOFTHE CARAPACE 

 O] \ SPID1 R \ :! V. i I) FROM \B0\ E. 

 THE W rERIOR Row OF EYES AP- 

 PEARS TO BE PRO( I R\ l D 



lateral, one at each end of the first row: and posterior lateral, one 

 at each end of the second row. Some writers refer to the 

 lateral eyes as the side eyes. 



The rows of eyes are frequently curved. When the lateral 

 eyes of a row are farther forward than the median eyes the row 



<)7 



