266 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH, 
The branchial formula of Astacus. 
Somites and Arthrobranchiz 
their Podo- —_—_~. Pleuro- 
Appendages. branchie. Anterior. Posterior. branchie. 
vi... Ofep)...0 1.0 .. 0 = 0 (ep.) 
vir .. 1 wid . 0 0 = 2 
Ix, ve re 0 = 3 
Ken wea OE ee | we 0 = 8 
xi. 1 Pea | rome 0 orr = 30r34+°r 
D4 | er | er ed La =3+r 
xl... 61 an oe r =3+r 
XIV. 0 . 0 .0 1 = 
6+ep+6 +5 + 14+20r3r7=18 + ep. + 2Qor3r. 
In Cambarus, the number of the branchie is reduced 
to seventeen by the disappearance of the last pleuro- 
branchia; while, in Astacoides, the process of reduction 
is carried so far, that only twelve complete branchie are 
left, the rest being either represented by mere rudiments, 
or disappearing altogether. 
The branchial formula of A stacoides. 
Somites and Arthrobranchiz, 
their Podo- ————__—- Pleuro- 
Appendages. branchie. Anterior. Posterior. branchiz. 
vi... O(epr) O .. O 0 = = O(ep.r) 
vill. ... 1 oes 0 0 = l+r 
Ix. J 1 0 0 = 2 
x. 1 is 1 r 0 = 2+ 
XI. 1 1 r 0 = 2+fr 
XI. 1 5 re 0 = 24+r 
xl. ... 1 5 Cre 0 = 2+r 
XIV, 0 eas 0 0 : a 
6+ep4r 5b+r4+04+4r41 = 12 4 epr.¢ 57, 
