Chilton. — On Astacus fluviatilis and Paranephrops setosus. 



true branchiae. 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 branchiae, but they 

 have no lamina or epipodite corresponding to that of Astacus, and many 

 though not all of the branchial filaments have hooked apices (pi. XXI., 

 fig. 5). Each of these six appendages also bears on its interarticular mem- 

 brane an ar thro bran chia. These correspond to the anterior arthrobranchia 

 of Huxley. They are all well developed, but are considerably smaller than 

 the podobranchise. 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 pleurohranchiae are attached to the 

 epimera of the four hindmost thoracic somites. They are all of moderate 

 size. 



These facts may therefore be tabulated thus : — 



Branchial Formula of Paranephrops setosus. 



Somites and their 



Podobranchise. 



Arthrobranchiae. 



Pleurobranehige. 





Appendages. 



Anterior 



Posterior 





VII. 



ep. r. 















ep. r. 



VIII. 



1 



1 











2 



IX. 



1 



1 



1 







3 



X. 



1 



1 



1 







3 



XI. 



1 



1 



1 



1 



4 



XII. 



1 



1 



1 



1 



4 



XIII. 



1 



1 



1 or r 



1 



4 or 3 + r 



XIV. 















1 



1 





6 + ep.r. 



+ 6 + 



5 or 4 + r 



+ 4 



21 + ep. r., or 

 20 + r + ep. r. 



The coxopoditic setse 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 Astacus. The seta? of which each tuft 



