28 



THE PINEAL ORGAN 



worms. In higher types of invertebrates it is found that one such pair 

 of eyes has become much more highly evolved and larger than the others, 

 and constitutes the functional lateral pair of eyes, whereas the other pair 

 or pairs remain small and simple in type. In some insects, e.g. the gad 

 fly (Tabanus) (Fig. 20), the faceted lateral eyes having increased enor- 

 mously in size and being directed forwards as well as laterally, have come so 

 near together that the area which gives origin and nutriment to the median 

 pair or pairs of eyes is encroached upon, and the whole area, including its 

 vessels and nerves, being reduced, the median eyes also become reduced in 

 size, approximated, and fused. It thus seems possible that a mechanical 

 and nutritive as well as a functional preponderance of one pair of eyes over 



c.hem. 

 <^y^,had^ jnt.br. 



nv.*' 



sp.n. 



Fig. 22. — Left Side of the Brain of Petromyzon, showing the Relations of 

 the Pineal Organs and the Habenular Ganglia. (After Ahlborn.) 



the other may come into play and contribute to the fusion and atrophy 

 of the less developed pair. In other invertebrates, e.g. some of the low- 

 type crustaceans, more particularly Daphnia, Polyphemus, and Leptodora 

 (Fig. 21), it is the lateral paired eyes which become fused. The under- 

 lying principle which is involved, however, is the same as in the fusion of 

 the median eyes. There is, with the adoption of a parasitic life, a general 

 reduction in the size of the animal and more particularly of the head, 

 and with the loss of the necessity for using the eyes along with atrophy 

 of the interocular tissues, the eyes are brought in contact with each other 

 and fuse into a single organ. 



The substitution of a single unpaired organ for the earlier paired 

 median eyes is exemplified also in the pineal organ of the vertebrates. 



