Cuiurox.— On Astacus fluviatilis and Paranephrops setosus. 159 
true branchie. Hence this epipodite must be looked upon as forming part 
of the respiratory organs. The next six appendages, viz., the second and 
third maxillipedes and the first four pairs of ambulatory legs, each bear a 
podobranchia. These are larger than any of the other branchim, but they 
have no lamina or epipodite corresponding to that of Astacus, and many 
though not all of the branchial filaments have hooked apices (pl. XXI., 
fig. 5). Each of these six appendages also bears on its interarticular mem- 
brane an arthrobranchia. These correspond to the anterior arthrobranchia 
of Huxley. They are all well developed, but are considerably smaller than 
the podobranchiw. To all these appendages except the first, the second 
maxilla, there is also the corresponding posterior arthrobranchia, These 
are all of small size, the largest being composed of comparatively few fila- 
ments, and they become smaller from before backwards. In fact the last 
one is almost if not quite rudimentary, though evidently subject to some 
variation, for, in one specimen, on the left side it was composed of a single 
short filament, while on the right the filament was longer and bore a short 
filament branching from it. Four pleurobranchi# are attached to the 
epimera of the four hindmost thoracic somites. They are all of moderate 
size. 
These facts may therefore be tabulated thus :— 
BrancuiaL Formuta or Paranephrops setosus. 
Somites "€ their Poldowaidhie a xus * 
| Anterior |Posterior| 
VII. ep. r. 0 0 0 ep. r 
VIII. 1 1 0 p 2 
IX. 1 1 1 0 š 
X. JE 1 1 0 8 
XL 1 1 1 i 4 
XII. 1 1 1 1 4 
—— 1 1 lorr 1 4or3 +r 
XIV. 0 0 0 1 i 
6+ep.rj+ 6 +)50r4+r + 4 i Ad ms or 
The coxopoditic sete which are found on the coxopodites of the four 
pairs of ambulatory limbs, and also of the third pair of maxillipedes, differ 
considerably from those of Astacus. They are few in number and much 
shorter and straighter than those of 4stacus. The sete of which each tuft 
