The Retina and Optic Ganglia in Decapods, especially in Astaciis. 1 1 
followiüg elements : two corneagen cells, fo u r cone-celLs, t wo distal 
retinular cells, one mdimentary and seven functional proximal reti- 
nular cells, making a total of sixteen cells. This number is not only 
characteristic for Astacus^ but, as I have previously attempted to show 
(Parker, 91, pag. 114), for all decapods, a conclusion that is supported 
by tbe recent investigations of Viallanes (92) on Palinurus and of 
Herrick (92) on Alpheus so far as tbey touch this question. The 
enumeration of the ommatidial cells in Astacus as given by Szcza- 
wiNSKA (91) Supports, except in two particulars to be discussed later, 
this same conclusion. 
The accessory pigment cells in Astacus fill the space 
between the proximal ends of the ommatidia, and extend from the 
distal surface of the basement membrane to the middle of the 
rhabdomes. Their nuclei occupy proximal positions (PI. 1 Figs. 1 
and 7, nl.pg\ and their pigment, like that in the corresponding cells 
of other decapods, has the quality of reflecting light. These cells, 
which were first described in Astacus by CARRii:RE (85, pag. 169), 
were also observed by Szczawinska (91, pag. 546), who believed 
that each omraatidium probably contained seven of them, since they 
seemed to alternate with the seven proximal retinular cells. The 
bodies that she figures (PI. 17 Fig. 9, pg S)^ however, do not re- 
present each a single cell but processes from cells ; and, as I ascer- 
tained by counting the nuclei, the number of these cells present for each 
ommatidium, though somewhat variable, was usually one, never seven. 
The two corneagen cells cover the distal end of the omma- 
tidium; their nuclei occupy the dorsal and ventral angles of this 
structure (PI. 1 Figs. 1 and 3, nl. crn) and are separated by a line, 
which, though not visible in the protoplasm of their cells, is well 
marked in the corneal cuticula, the product of these cells. This 
line extends from the anterior to the posterior angle of the facet, 
and represents very probably the boundary between these two cells. 
My observations confirm those of Carri^^re (89, pag. 225) and of 
Szczawinska (91, pag. 541), according to whom there are only two 
corneagen cells for each ommatidium; Reichenbach’s (86, pag. 91) 
previous enumeration of four such cells in Astacus is, without much 
question, erroneous. 
The four cone-cells are in contact with the proximal surface 
of the corneal hypodermis, and extend from this level to the basement 
membrane. When seen from the side (PI. 1 Fig. 1), they present 
four distinctly marked regions; first, a relatively thin distal zone 
