BRANCHIAL FORMULA. 265 
This tabular ‘‘ branchial formula” exhibits at a glance 
not only the total number of branchie, but that of each 
kind of branchia; and that of all kinds connected with 
each somite; and it further indicates that the podo- 
branchia of the first thoracic somite has become so far 
modified, that it is represented only by an epipodite, with 
branchial filaments scattered upon its surface. 
In Palinurus, these branchial filaments are absent and 
the branchial formula therefore becomes— 
Somites and Arthrobranchiz. 
their Podo- —_—~ Pleuro- 
Appendages, branchie. Anterior. Posterior, branchiz. 
VIL... 0 (ep.) 0 0 Oo = 0 (ep.) 
VIIL. 1 oe 1 0 0 = 2 
Ix, 1 1 1 bse 
x. 1 1 1 0 = 3 
XI. 1 1 1 1 = 4 
xii. 1 1 1 l= 4 
XIII 1 1 1 1 = 4 
XIV. 0 0 0 1 = 1 
6+ep + 6 + 5 + 4 = 21 + ep. 
In the lobster, the solitary arthrobranchia of the eighth 
somite disappears, and the branchie are reduced to twenty 
on each side. 
In Astacus, this branchia remains; but, in the English 
crayfish, the most anterior of the pleurobranchie has 
vanished, and mere rudiments of the two next remain. 
It has been mentioned that other Astacit present a 
rudiment of the first pleurobranchia. 
