The Retina and Optic Ganglia in Decapods, especially in Astacus. 7 
and the part measured would be subjected to tbe same distortion. 
The metbod bas been tested by comparing its results with actual 
enumerations. In all cases the estimated number has not difFered 
from the actual number by more than \% of the total. Thus, in 
one case, the estimated number of facets was 326.4, the counted 
number 324.75, a difference of approximately a half per cent. This 
seems to me to justify the employment of the method. 
3. Greneral Structure of Optic Stalks. 
The neiTOus Organs contained within the optic stalk of the 
crayfish are so complex in structure that it will add considerably 
to the clearness of the following account if, in the beginning, a few 
words are devoted to the shape and position of the stalks them- 
selves. Each stalk has somewhat the form of a short cylinder 
movably attached by its proximal end to the animahs body and 
terminated distally by a nearly hemispherical surface. This surface 
is for the most part occupied by the retina and may be called the 
distal surface of the stalk; a line connecting its centre with 
the centre of the proximal end of the stalk defines the axis of this 
structure. As the stalk cannot be rotated around this axis, the sur- 
face of its cylindrical portion may be subdivided as follows: when 
its axis Stands at right angles to the niedian plane of the animal, 
that portion of its cylindrical surface which faces dorsally may be 
called its dorsal surface; that which faces anteriorly, its anterior 
surface, etc. Thus, in addition to its distal surface, the stalk may 
be said to have a dorsal, a ventral, an anterior, and a 
posterior surface. These four surfaces border, of course, on the 
distal surface, whose periphery may therefore be divided into four 
corresponding Segments — dorsal, ventral, anterior, and posterior. 
This method of subdividing the surface of the stalk might at first 
seem rather arbitrary ; but, as a matter of fact, the four surfaces 
of the cylindrical portion are easily recognised, since the stalk is 
not strictly cylindrical, but flattened dorsoventrally. The terms 
mentioned in this paragraph have been found convenient in describ- 
ing the crayfis¥s eye, and their usage as defined above will be 
adhered to in the following pages. 
The nervous structures contained within the stalks have 
received such a variety of names that an accepted nomenclature for 
them can hardly be said to exist. Although no single set of terms 
