228 



TRILOBITA 



Three types of compound eye have been recognised in 

 Trilobites ^ — holochroal, prismatic, and schizochroal. 



1. In the holochroal eye (Fig. 139, A, B) the lenses are 

 globular or biconvex and close together, so that the cornea is 



Fig. 139. — Eyes of Trilobites. (After Liudstrom. ) A, B, Sphaerophthalmus alafus, Ang. 

 Upper Cambrian. Vertical and horizontal sections, x 100. C, Asaphus fallax, 

 Dalm. Horizontal section, x 60. D, NiJeus armadillo, Dalni. Vertical section, 

 X 60, a, prismatic lenses ; h, cuticle ; c, part of free cheek. E, Dalmaniles 

 vulgaris. Salt. Part of eye, x 30. F, Dalmanites imbricaiulus, Ang. Vertical 

 section of eye, with a part of the free cheek on the left, x 60. G, H, Harpes 

 ■cittutHs, Barr. G, The two lenses of one eye, x 8 ; H, vertical section of the same, 

 x60. 



Examples of this are seen in 



continuous over the entire eye. 

 BrorUeus and Sphaero])Jifh(diii us. 



^ Lindstroni, " Visual Organs of Trilobites," Svenska Vet. Akad. Handl. xxxiv., 

 1903. Exner, Physiol, d. facett. Aaijenv. Krebscn u. Insecten, 1891, p. 34, pi. ii. 

 liss. 18, 19. 



