THE BRANCHIE OF PENZUS, 267 
As these formule show, those trichobranchiate crus- 
tacea, which possess fewer than twenty-one complete 
branchie on each side, commonly present traces of the 
missing ones, either in the shape of epipodites, as in the 
case of the podobranchie, or of minute rudiments, in the 
case of the arthrobranchie and the pleurobranchie. 
In the marine, prawn-like, genus Peneus (fig. 78, 
Chap. VI.), the gills are curiously modified trichobran- 
chie. The number of functional branchie is, as in the 
lobster, twenty; but the study of their disposition shows 
that the total is made up in a very different way. 
The branchial formula of Peneus. 
Somites and Arthrobranchiz. 
their Podo- —_——~ Pleuro- 
Appendages. branchie. Anterior. Posterior. branchiz. 
vi... O(ep) «. 1 oo... O . OO = L+tep. 
VIII. ... 0 (ep.) 1 1 1 = 38+ ep. 
Ix, 0 (ep.) 1 1 1 = 8+ ep 
x. 0 (ep.) 1 1 1 = 8+ep 
xI. O(ep.) .. 1 1 1 = 8+ ep 
XI... Ofep.) .. 1 1 1 = 8+ep 
xl... 60 1 1 Ll. S428 
xIv. .. O 0 0 1 = 1 
O+Gep. + 7 + 6 + 7 = 204 6ep. 
This case is very interesting; for it shows that the 
whole of the podobranchie may lose their branchial charac- 
ter, and be reduced to epipodites, as is the case with the 
first in the crayfish and lobster, and indeed in most of 
the forms under consideration. And since all but one of 
the somites bear both arthrobranchie and pleurobranchia, 
