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TEXT-BOOK OF ENTOMOLOGY 



of minute fibrillae, the neurospongiuin of Hickson. In the young 

 cockroach (Periplaneta) the optic nerve separating the cerebral 

 ganglion from the opticon is much longer in proportion than it is in 

 the adult blow-fly. 



The second ganglionic swelling (epiopticon, Fig. 261, c.op) is 

 separated from the opticon by a tract of fine nerve-fibrils, which 

 partially decussate ; at the decussation two or three larger nerve- 

 cells may be seen. It also contains a few scattered nerve-cells (w.c). 



The third ganglionic swelling (peri- 

 opticon, p.op} is separated from the 

 others by a bundle of long optic 

 nerve-fibrils, which cross one another. 

 It is composed of a number of cylin- 

 drical masses of neurospongium 

 arranged side by side (Fig. 261, 

 p.op). Between these elements of 

 the periopticon, which do not seem 

 to bear any relation to the number 

 of ommatidia, a single nerve-cell is 

 very frequently seen. The periopti- 

 con does not occur in Periplaneta and 

 Nepa (Hickson). The three optic 

 ganglia thus described, together with 

 the cerebral ganglia, are surrounded 

 by a sheath of densely packed nerve- 

 cells. 



FIG. 266. Periopticon and terminal 

 anastomosis of Agrion, showing- the char- 

 acter of the elements of the periopticou 

 (Ji.cj'\ and the structure of the terminal 

 anastomosis (.a). 1. The first layer of the 



Bearing in mind the fact that the re- 

 tinulfe are the nerve-end cells of the fibres 



terminal anastomosis consisting of a plexus pass j ng through the periopticon, it will be 

 of nlmls and nerve-cells (.c). '2. The sec- 

 ond layer, in which the fibrils are collected 

 tog-ether in bundles. 3. The final optic 



plrxiis and nerve-cells. 4. The layer in 



well to read the following account, by Hick- 

 son, of the terminal anastomosis of the 



which the optic fibrils are collected in bun- optic fibrils in the periopticon of Agrion 



dlestobedistributed to theretinuUe(r); b.m, hifiirrntnm aiirl to pviirmiP hi kptHi 

 basal membrane. -After Hickson. OlJUrcaiWm, aim IO examine IHS bKeiCll 



(Fig. 206): 



" The terminal anastomosis of Agrion may be conveniently divided into four 

 regions. First the region (1) lying nearest to the periopticon in which the 

 nerve-cells are numerous, and the fibrils leaving the periopticon form a compli- 

 cated plexus; the region (2) next to this, in which the fibrils have collected into 

 bundles separated by spaces occupied by very thin-walled tracheae in which 

 there are no spiral markings, and lymph-spaces; next, the region (3) in which 

 the fibrils form a final plexus, and in which there are again a considerable num- 

 ber of nerve-cells ; and, lastly, the region (4) in which the fibrils are again col- 

 lected into bundles, separated by spaces containing trachese, which perforate the 

 basement membrane to supply the retinulaa." 



It would seem as if the decussation of the optic nerve-fibrils were a matter of 



