128 



LARVAL OCELLI AND THE PARIETAL EYE. 



are two transitory sense organs which appear to represent the thoracic series of 

 Limulus. They disappear before hatching, after contributing an important mass 

 of ganglion cells to the pedal nerves. 



The series of procephalic and thoracic sense organs just described should not 

 be confused with the segmentally arranged gustatory organs, which belong to a 

 different system, and which are always located on the median side of the base of 

 the appendages. 



After this preliminary survey, we may consider the several organs under dis- 

 cussion in more detail. 



III. The Ocelli of Insects. 



A very primitive and suggestive condition is seen in Acilius, where the early 

 history of the ocelli is best known. Here the cephalic lobes are clearly divided 

 into three segments, each one containing a segment of the brain, one of the optic 

 ganglion, and one of the optic plate. (Fig. 14.) Three deep infoldings, iv, ^-'j 



Fig, 90. — The ocellus of an insect larva, Acilius (eye V). This ocellus looks forward and outward. 



form on the median side of the plate, carrying the three-lobed optic gangHon be- 

 low the surface. The openings soon close, without the formation of a paHal fold 

 like that which covers the whole forebrain in the scorpion. 



The ocelli are formed by separate, pit-like infoldings of the optic plates, the 

 retina forming from the bottom of the pits and the dioptric apparatus from the 

 lips of the closed vesicles. (Figs. 90-91 and 102.) 



At the close of larval life, the ocelli break away from the surface ectoderm 



