1878.] AND DISTRIBUTION OF THE CRAYFISHES. Ill 



gills or " phyllobranchiae," which are met with in a large number of 

 other Crustacea. The whole of the Macrurous Podophthalmia, 

 excepting the genera Gebia aud Callianassa, the Prawns, the 

 Shrimps, and the Mysidae, have trichobranchiae. 



In the Mysidae the branchiae are rudimentary or absent, and in 

 the Euphausidse and Penseidse they are peculiarly modified. In the 

 Prawns aud Shrimps, in Gebia and Callianassa, in all the Auomura 

 and Brachyura, the gills are phyllobranchiae. 



Thus the Podophthalmia or Thoracostraca (to use the convenient 

 name proposed by Prof. Claus) are divisible in respect of the structure 

 of their respiratory organs into three groups, which may be termed 

 Abranchiata, Trichobranchiata, and Phyllobranchiata. 



Among the trichobranchiate Podophthalmia, the Euphausidae 

 possess no other than podobranchiae \ These are mere respiratory 

 plumes presenting no differentiation even into base and stem. Ail 

 the rest of the Trichobranchiata have arthrobranchiae, either with or 

 without functional podobranchise and pleurobranchiae. Among 

 those which possess all three kinds of brauchiae, the Parastacidae and 

 the Palinuridae are highly exceptional among the Thoracostraca in the 

 absence of the appendages of the first abdominal somite in both sexes. 

 They further, as a rule, possess 21 branchiae {pdb. 6, arb. 1 \,plb. A), 

 though the number is, in some cases, reduced by the suppression of 

 more or fewer of the arthrobranchiae and pleurobranchiae. 



In most, if not all the other Trichobranchiata, the first abdominal 

 appendages of the males are present and specially modified. Among 

 these, the Potamobiidae are characterized by the imperfect division 

 of their podobranchiae into a proper branchial and an epipoditic 

 portion. 



In Homurus and Nephrope, Axius aud Thalasaina, the podo- 

 branchias are completely differentiated, from their bases onward, into 

 a proper branchial and an epipoditic portion. In this condition the 

 podobranchia is usually described as an epipodite, to the base of 

 which a brauchia is attached. 



In Homarus the branchial filaments are numerous and multiserial, 

 and the branchial formula is : — ■ 



Somites ■„ -, Arthrobrancliia:. T ->t 



-, ,, ■ Fodo- Pleuro- 



aud their , , . ^ — * .. , , . 



j branchiae. A , -,■> l ■ branchiae. 



appendages. Anterior. Posterior, 



VII (ep) = (.ep) 



VIII 1 = 1 



IX 1 I 1 = 3 



X 1 1 1 = 3 



XI 1 1 1 1 = 4 



XII 1 1 1 1 = 4 



XIII 1 1 1 1=4 



XIV 1 = 1 



6 + ep + 5 + 5 + 4 = 20 + ep. 



1 Possibly some of the branchial plumes in Sergestes may be attached to the 

 arthrodial membranes. A critical examination of the species of Scrgesfes in 

 reference to this point would probably yield interesting results. 



