26 



THE PINEAL ORGAN 



axial cell continuous with a single nerve fibre, the whole constituting an 

 ommatidium ' or unit of the composite eye with its separate lens or corneal 

 facet. 



As previously mentioned, the fusion of two median eyes to form a 

 single median eye is common in certain Insecta and Crustacea, and is 

 frequently attended by regressive changes with diminution or loss of 

 function ; and it will be useful at this stage of our description to consider 

 the possible causes leading to (i) the approximation of a pair of eyes, and 

 (2) their fusion and degeneration. 



-LE 



Fig. 20. 



Paired Lateral Eyes and Single Median Eye of a Gad Fly(Tabanus). 



(After Lang.) 



ant. : antenna. mx. 1 : First pair of maxillae. 



F. oc. : frontal ocellus. mx. 2 : proboscis which is formed 



h. ph. : hypopharynx. from the labrum which encloses 



Ibr. : labrum. the mandibles and maxillae. 



L.E. : lateral or faceted eye. P.m. : palp o the first pair of 



Mnd. : mandible. maxillae. 



If we examine the simple, pigmented eyes of certain species of flat 

 worm, e.g. the Planaria, we find that in some the eyes are arranged in a 

 semicircle around the front and sides of the head, the eyes being on the 

 dorsal aspect of the head and near its edge (Fig. 19) ; in other species in 

 addition to the eyes round the margin of the head there is a pair, some- 

 times two pairs, of eyes placed medially and farther back than the others 

 (Fig. 19, A) ; while in a third group the marginal row of eyes is absent 



1 Ommatidium : oppia, eye ; owjcctiov, diminutive of eye. 



