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Show 1878.] AND DISTKIBUTION OF THE CRAYFISHES. /// gills or " phyllobranchia?," which are met with in a large number of other Crustacea. The whole of the Macrurous Podophthalmia, excepting the genera Gebia and Callianassa, the Prawns, the Shrimps, and the Mysidae, have trichobranchia?. In the Mysidae the branchiae are rudimentary or absent, and in the Euphausidae and Penseida? they are peculiarly modified. In the Prawns aud Shrimps, in Gebia and Callianassa, in all the Anomura and Brachyura, the gills are phyllobranchia?. 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 Euphausida? possess no other than podobranchia?1. 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 podobranchiae and pleurobranchia?. Among those which possess all three kinds of branchiae, the Parastacida? and the Palinurida? 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 l,plb. 4), 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 Homarus and Nephrops, Axius and Thalassina, the podobranchiae 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 branchia is attached. In Homarus the branchial filaments are numerous and multiserial, and the branchial formula is :- Somites podo Arthrobranchiae. pieuw)_ and their brauclliiE-r7~ i~- "*"T , -~" branchia?. appendages. Anterior. Posterior. vu <UeP) ° ° ° = ° (eP) VIII 1 0 0 0 = 1 IX 1 1 1 0 = 3 X 1 1 1 0 = 3 XI 1 1 1 1 = 4 XII 1 1 I 1 = 4 XIII 1 1 1 1 = 4 XIV 0 0 0 1 = 1 6 + ep +5 + 5 + 4 = 20 + ep. 1 Possibly some of the branchial plumes in Sergestes may be attached arthrodial membranes. A critical examination of the species of Sergestes in reference to this point would probably yield interesting results. |