THE EYE AND OPTIC TRACT OE INSECTS. 
251 
here formed of a continuous neurospongium, perforated by numerous thin- 
walled tracheal vessels. The tracheal vessels passing through the periopticon 
are only represented on the right-hand side of the figure, they pass from the 
thick-walled spirally-marked branching tracheae ( T ) behind the periopticon to 
similar tracheae situated in front of it. The tracheae passing in front of the 
periopticon are supplied by the large tracheal trunk, Tt. v whereas those 
passing behind are supplied by the smaller tracheal trunk, Tt. 2 Rh. The 
rhabdom. r ■ The retinulse. b. m. The basilar membrane. pg. 3 . The internal 
pigment-cells. 
Eig. 24. — Transverse section through the periopticon of Eristalis 
lupin us, X 600 diam. t. v. The bundles of tracheal vessels perforating 
the neurospongium of the periopticon, p. op. 
Fig. 25. — A portion of the epiopticon of a young bee, with nerve-fibrils 
proceeding from it and the nerve-cells connected with them. The neuro- 
spongium of the epiopticon ( e . op.) is composed of a large number of very fine 
fibrillae, forming a dense meshwork and combining together in bundles to form 
the nerve-fibrils (Nf.) which pass out of it. The nerve-cells (». c.) consist of a 
large nucleus surrounded by a delicate investment of cell protoplasm, which 
gives off fine branches communicating with the neurospongium and the fibrils. 
Fig. 26. — A small portion of the epiopticon of an adult Aeschna 
grand is, showing the character of the neurospongium and the relative size 
and position of the nerve-cells ( n . c.) and nerve-fibrils {iff.). 
Fig. 27. — Ganglion-cells from the brain of an adult male Periplaneta 
orientalis. a. A tripolar ganglion-cell. b. A bipolar cell. c. An 
apolar cell. 
Fig. 28. — Portion of the basilar membrane of the eye of Agrion bifur- 
cat um, showing the perforations, the larger ones for the tracheal vessels and 
the smaller ones for the terminal optic fibrils, r. Retinulae. t. a. Terminal 
anastomosis. 
Figs. 29 — 32. — Diagrammatic figures of the four principal varieties of 
periopticon met with in the Hexapoda. The nerve-cells are omitted. 
Fig. 29. — The condition in Blatta in which the fibrils simply split up and 
anastomose. 
Fig. 30. — The condition in Nepa and the developing bee, in which the 
anastomosis is complicated by fine looped and transverse anastomoses. 
Fig. 31. — The condition in Musca in which the fibrils split up into their 
ultimate fibrillse and these anastomose to form a true neurospongium. 
Fig. 32. — The condition in Crustacese in which the elements of the peri- 
opticon have fused to form one single continuous neurospongium. 
Fig. 33. — Diagram of the * Acone ’ without any refracting bodies. 
Fig. 34. — Diagram of the ‘ Pseudocone,’ with the nucleus internal and the 
refracting bodies external. 
Fig. 35. — Diagram of the * Eucoue,’ with the nuclei external and the 
refracting bodies internal. 
