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 Lindstrdm.) A, B, Splmerophthalmus alatus, kyi%. 

 Upper Cambrian. Vertical and horizontal sections, x 100. C, AsapJms falfax, 

 Daltn. Horizontal section, x 60. D, A'iteus annadillo, Dalm. Vertical section, 

 X 60, a, prismatic lenses ; 6, cuticle ; c, part of free cheek. E, Dalmanites 

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

 section of eye, with u, part of the free cheelc on the left, X 60. G, H, Harpes 

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

 x60. 



continuous over the entire eye. Examples of this are seen in 

 Bronteus and Sphaerophthalmus. 



' Lindstrtim, " Visual Organs of Trilobites," Svenska Vet. Akad. Ilandl. xx.\-iv. , 

 1903. Exner, Physiol, il. facett. Atujen v. Krebsen u. Insccten, 1891, p. .34, pi. ii. 

 ligs. 18, 19. 



