PARIETAL EYE OF THE SCORPION. 



I2 9 



and become lodged deep in the head, on the surface of the optic ganglia, where 

 they degenerate. 



The frontal ocelli are new formations, usually appearing at the beginning of 

 the metamorphosis, and differing from the larval ocelli in their mode of develop- 

 ment, time of appearance, and relation to the brain. 



IV. THE PARIETAL EYE. 



Parietal Eye of the Scorpion. 



The development of the parietal eye in the scorpion and spiders furnishes 

 the best picture of the process by which ocelli are carried into the brain 

 chamber to form a true parietal eye like that in 

 vertebrates. 



The evolution of the brain chamber and 

 the parietal eye is essentially the same in scor- 

 pions and spiders. (Figs. 15, 20, 21.) I will 

 describe the condition in the former. 



The cephalic lobes soon divide into three 

 segments that have a very constant and charac- 

 teristic form in the arachnids. (Fig. 15.) One 

 may distinguish the centrally located brain neu- 

 romeres, br.*~ 3 , two prominent optic ganglia, and 

 a marginal plate, with deep infoldings between it 

 and the ganglia. 



The whole of the first segment forms a dark 

 infolded band, extending across the anterior 

 margin of the cephalic lobes. From it is formed 

 the olfactory lobes (organ stratifie of St. Remy). 



The lateral lobe of the second segment forms 

 the optic ganglion of the median eyes, p.e.g., 

 and the one behind forms the ganglion of the 

 lateral eyes, l.e.g. 



Between the two ganglia and the lateral margin of the cephalic lobes are two 

 infoldings, the floor of which is formed by the lateral portions of the optic gan- 

 glia, iv 2 -iv 3 . 



The median ocelli will develop from the extreme lateral margins of the 

 cephalic lobes, opposite the second pair of infoldings, and the lateral ocelli oppo- 

 site the third pair. The ocelli, however, are not visible till later. 



The arachnid cephalic lobes are clearly comparable with those of Acilius, the 

 principal differences lying in the union of the parts of the first segment to form the 

 olfactory lobe, and the small size and late appearance of the ocellar placodes. 



9 



FIG. 91. The ocellus of an insect larva, 

 Acilius (eye 7). This eye looks directly 

 upward. 



