Comparative Anatomy of the Eye. 447 



They number in the eye of the ant about 50, while in the eye of the 

 butterfly as many us 3, 650 have been counted. According to Agassiz 

 some compound e}-es have 25,000 facets. Behind these facets are 

 crystalline rods enveloped in pigment connecting with nervous filaments 

 through a pigment layer. These filaments connect inwardly with a 

 ganglionary expansion of the optic nerve. A large number of six- 

 footed insects when adult possess these compound eyes, two in number, 



FIG. 179. Diagram of Compound facet- 

 ed Eye of Libellula (Dragon Fly.} 



C'. Cornea. 



K. Crystalline Cone. 



P. Pigment. 



#. Nerve Rods. 



A'6. Layer of Fibres. 



Gz. Layer of Ganglion Cells. 



Rf. Retinal Fibres. 



Kfc. Crossing of Fibres. 



The faceted Eye occurs in Insects and 

 Crustaceans ; and enables the animal to 

 take in a wide field of vision without 

 moving. 



and also three of the ocelli, the ocelli 

 being situated between and a little 

 * n a( *vance * tlJ e compound eyes. 

 179 The number three, however, is in- 



conclusive. The fact is, that under the influence of bilateralism there 

 are originally four ocelli. The anterior single ocellus is formed by the 

 fusion of two original Ocelli as is seen in the development of the pupa 

 of the bumble-bee (Bombus). Packard holds, that the articulate eyes 

 are simply modified dermal sense cells. ' ' In the embryos of all the 

 insects yet examined the eyes are groups of specialized cells of the skin 

 which grow out on the upper or tergal side of the same segment which 

 bears the Antenae. " l They are not however confined to this segment 

 in all the Articulates. In the king crab (Limulus) one pair of ocelli 

 are found on the first segment, and the compound eyes are on the back 

 of the third segment. Among the worms some, as the Polyophthalmus, 

 have eyes on every segment, while some of the Planarians have them 

 scattered irregularly over the body. In Spiders the number is usually 

 eight and they are in a cluster on the front end of the back. In some 

 of the Millipedes, the Pill-bugs for example, the eyes are collected in 

 groups. These are in a sort of transition state between the single 

 simple eye and the compound eye. In the Nereis, a marine worm, there 

 are four eyes, of a very simple pattern ; consisting of a black cup- 

 shaped membrane or choroid which contains a small, white, opaque body 

 flattened in front and rounded behind, and which is connected with the 

 optic nerve. In front it is covered with a layer of epithelial cuticle. 

 The eyes of the Craw-fish are compound like the eyes of insects but are 

 set up on eye stalks. These are flexible tubes which can be turned in 



1 Packard On Insects 19. 



