The Eetina and Optio Ganglia in Decapods, especially 
in Astacus. 
By 
Gr. H. Parker, 
Instructor in Zoölogy, Harvard College, Cambridge, Mass., U. S. A. 
With Plates 1 — 3. 
1. Introduction. 
The recent discoveries in the finer structure of the nervous 
System have been so numerous and so important that we may well 
regard the present as a period of more than nsual progress in this 
direction. The advancement thus far made has been rendered possible 
mainly through the employment of two new methods of histological 
investigation: Golgi’s method for impregnating nervous elements 
with salts of silver and Ehklich’s method for staining living nervous 
elements with methylen blue. Both these methods have yielded 
extraordinary results, but they have been used chiefly in elucidating 
the finer structure of the nervous System in vertebrates, though, as 
the investigations of Biedermann , von Lenhossek, Retzius, and 
others show, the invertebrates have by no means been neglected. 
It was my purpose in undertaking the present studies to attempt 
an application of these methods to the solution of some problem in 
the Organisation of the nervous System in invertebrates, and, being 
already somewhat familiär with the structure of the retina in crus- 
taceans, I was led to take up the study of the. optic ganglia in 
these animal s. 
The results that are recorded in the following pages represent, 
in part, the work of two years during which I held an appointment 
Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. 12. 1 
