362 THE PINEAL ORGAN 



group, namely median eyes, or speak of Patten's second group as the 

 Entomostracan eye, and the third group as the hexapod type or frontal 

 eyes of insects. 



Patten contrasted the principal characters of his four types in the 

 following manner : 



ant 



n.pg.c. 



n 6-q.f.o 



n.en.pe- 



Fig. 250.- 



(Redrawn 



-Triplacodal Parietal Eye of Branchipus 

 from Patten.) 



A — Axial section from front to back. 



B — Median sagittal section. The three-lobed vesicle consists of right and left 

 ectoparietal eyes and a single (probably bilateral) entoparietal eye. The 

 cavity is completely cut off from the exterior in the adult animal. The 

 distal ends of the retinal cells, which contain imperfectly formed rods, are 

 turned towards the lumen of the cavity. 



ant. : anterior end. n. ec. p.e. : nerve of ectoparietal eye. 



ec. p.e. : ectoparietal eye. n 



ect. p. : ectodermal pit. ;/ 



en. p.e. : entoparietal eye. 



f.o. : frontal organ. 



g.c. : ganglion cell. 



n.p t 



post 



en. p.e. : nerve of entoparietal eye. 

 & g.f.o. : nerve and ganglion of 

 frontal organ. 

 c. : nucleus of pigment cell, 

 posterior end. 



i. " The larval ocelli (Fig. 78, Chap. 11, p. 117), of which there may 

 be six pairs. These are present in the active larvae of most insects, 

 but disappear during the metamorphosis (Coleoptera, Lepidoptera, 

 Neuroptera, Hymenoptera). They are cup-like infoldings of the ecto- 

 derm, with upright or horizontal retinal cells or rods." 



" In the insects the retinal cells are never completely inverted, and 



