144 



THE PINEAL ORGAN 



thin layer of ectoderm ; if the dorsal part of this and a small part of the 

 brain be removed, the eye can be seen lying in a blood space just dorsal 

 to the brain. It has the appearance of a black X which is slung on to the 



Fig. 105. — Paired Median Eyes in Nauplius of Decapod Carididae. (After 



Margaret Robinson.) 



A : Median eyes of Pandalus annulicornis, viewed from dorsal aspect after removal 

 of the rostrum. The pigment appears as an X-shaped mass between two 

 lateral groups of nerve-end cells. B : Transverse section through region of 

 median eyes of Virbius varians, showing ingrowth of suspensory ectodermal 

 cords. C : Transverse section farther back than B, showing the central 

 mass of pigment cells, with the nerve-end cells on either side. A median 

 ventral group of nerve-end cells is also present. D : Diagrammatic hori- 

 zontal section through brain of Virbius varians. The chitinous roof which 

 covers the median eyes is not shown in the sections B and C. 



ant. 1 and ant. 2 : first and second 



antennas. 

 bl. c. : blood corpuscle. 

 bl. s. : blood space. 



comm. : circumoesophageal commissures. 

 ect. : ectoderm. 

 end. : endoderm. 

 g. br. : nuclei of ganglion cells of brain. 



g.c. : ganglion cell. 



I.e. : lateral eye. 



m. : muscle. 



n.c. : ectodermal cord. 



op. n. : optic nerve. 



p. : pigment. 



p. corp. : pigment corpuscles. 



p.m.e. : pigment mass of median eye. 



ectoderm by two slender threads, which swell out in the concavities of the 

 X and then narrow again as they approximate to each other. 



" The X consists of two large pigment cells, and the supporting strings 



