268 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
the suggestion arises that each hypothetically complete 
thoracic somite should possess four gills on each side, 
giving the following 
Hypothetically complete branchial formula. 
Somites and Arthrobranchiz. 
their Podo- —_—"“—"~ Pleuro- 
Appendages. branchiz. Anterior. Posterior. branchiz. 
VII. 1 eg eg), Od od 
VIII. 1 1 1 1 = #4 
Ix. 1 1 1 1 = 4 
x. 1 Z 1 1 aa. 
xL 1 1 1 1 = 4 
XII. 1 1 1 1 = 4 
XIII 1 1 1 1 ne) 
XIV. 1 1 1 1 = 4 
8 + 8 + 8 + 8 = 382 
Starting from this hypothetically complete branchial 
formula, we may regard all the actual formule as pro- 
duced from it by the more or less complete suppression 
of the most anterior, or of the most posterior branchie, 
or of both, in each series. In the case of the podo 
branchie, the branchie are converted into epipodites; in 
that of the other branchiz, they become rudimentary, or 
disappear. 
In general appearance a common prawn (Palemon, 
fig. 71) is very similar to a miniature lobster or crayfish. 
Nor does a closer examination fail to reveal a complete 
fundamental likeness. The number of the somites, and 
of the appendages, and their general character and dispo- 
