THE ANTENNULES AND THE EYESTALKS. 173 
The antennule (fig. 48, B) has a three-jointed stem 
and two terminal annulated filaments, the outer of which 
is thicker and longer than the inner, and lies rather above 
as well as external to the latter. The peculiar form of 
the basal segment of the stem of the antennule hag already 
been adverted to (p. 116). It is longer than the other 
two segments put together, and near the anterior end 
its sternal edge is produced into a single strong spine (a). 
The stem of the antennule answers to the protopodite of 
the other limbs, though its division into three joints is 
unusual; the two terminal annulated filaments represent 
the endopodite and the exopodite. 
Finally, the eyestalk (A) has just the same structure 
as the protopodite of an abdominal limb, having a short 
basal and a long cylindrical terminal joint. 
From this brief statement of the characters of the appen- 
dages, it is clear that, in whatever sense it is allowable to 
say that the appendages of the abdomen are constructed 
upon one plan, which is modified in execution by the 
excess of development of one part over another, or by the 
suppression of parts, or by the coalescence of one part 
with another, it is allowable to say that all the apzen- 
dages are constructed on the same plan, and are modified 
on similar principles. Given a general type of appendage 
consisting of a protopodite, bearing a podobranchia, an 
endopodite and an exopodite, all the actual appendages 
are readily derivable from that type. 
