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SYDNEY J. HICKSON. 
and completely in Eristalis, Bombyx, and the Crustacea. In 
Eristalis the peri-opticon is traversed by a number of delicate 
tracheal vessels. 
The terminal optic anastomosis of Nepa is more complicated 
than it is in Periplaneta, and seems to be an intermediate 
stage between the simple anastomosis and the true peri-opticon 
of Musca. 
A similar series of intermediate stages between the simple 
anastomosis and a true peri-opticon has been traced in the 
development of these parts in the Bee. 
The development and comparative anatomy of the peri- 
opticon of insects is interesting, as it may indicate the mode in 
which central ganglia were first formed from primitive nerve- 
fibrils and cells. 
My researches seem to me to corroborate the opinion of the 
majority of previous investigators, that the retinulse are the 
true nerve-end cells. 
My researches were carried on entirely in the morphological 
laboratory at Oxford, and I have to thank Professor Moseley 
for much valuable help and advice, and to Professor Lankester 
for many valuable suggestions. 
§ 10. Bibliography. 
The following list does not pretend to be a complete biblio- 
graphy of the subject, but it contains the most important of 
the works I have consulted in preparing this memoir. For 
further information T must refer to Grenadier (7). 
1. Balfour, F. M.— “ Address to the Department of Anatomy and Physio- 
logy,” ‘ British Assoc. Report,’ 1880. 
2. Berger, E. — ‘ Untersuchungen iiber den Bau des Gehirns undder Retina 
der Arthropoden,’ Institut. Wien, i, Heft 2, 1873. 
3. Carriere, J. — “ On the Eyes of some Invertebrata,” * Quart. Journ. 
Micr. Sci.,’ vol. xxiii, p. 673. 
4. Ciaccio, M. G. V. — “De l’oeil des Dipteres,” ‘Journ. de Zoologie,’ v, p. 
312, from the ‘ Rendiconto Acad. Scienze Instituto di Bologna,’ 
1875-6, p. 99. 
