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 has 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 annulate.d filaments represent 

 the endopodite and the exopodite. 



Finall}^ the ej^estalk (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 ajDpendages 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 appen- 

 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. 



