The Retina and Optic Ganglia in Decapods, especially in Astacus. 7 
and the part measured would be subjected to the same distortion. 
The method has been tested by compariag its results with actual 
enumerations. In ali 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 difiference of approximately a half per cent. This 
seems to me to justify the employment of the method. 
3. General Structure of Optic Stalks. 
The nervous 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 animaFs 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 
distai surface of the stalk; a line connecting its centre with 
the centre of the proximal end of the stalk defìnes 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 median plane of the animai, 
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 ta its distai surface, the stalk may 
be said to bave a dorsal, a ventral, an anterior, and a 
posterior surface. These four surfaces border, of course, on the 
distai 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 
geem 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 bave been found convenient in describ- 
ing the crayfìsh's eye, and their usage as defined above will be 
adhered to in the following pages. 
The nervous structures contained within the stalks bave 
received such a variety of names that an accepted nomenclature for 
them can hardly be said to exist. Although no single set of terms 
