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 



Hi/pothetically complete branchial formula. 



8 + 8 + 8 + 8 = 12 



Starting from this hypothetically complete branchial 

 formula, we may regard all the actual formulae as pro- 

 duced from it by the more or less complete suppression 

 of the most anterior, or of the most posterior branchiae, 

 or of both, in each series. In the case of the podo- 

 branchise, the branchiae are converted into epipodites ; in 

 that of the other branchiae, they become rudimentary, or 

 disappear. 



In general appearance a common prawn {Palcemon, 

 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- 



