OCR Text |
Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. 783 of De Haan), if we exclude the Penaeida?, constitute a natural assemblage, to which I will apply the name of " Caridomorpha." They are all eminently Macrurous; and the characteristic feature of their branchial system is the predominence of the pleurobranchiae, and the concomitant diminution in the number and the importance of the arthrobranchia? and of the podobranchia?. In fact, so far as I am aware, there are never any traces of the latter except upon the maxillipedes. In both Palcemon and Crangon I find five pleurobranchia? attached to the posterior thoracic somites, from the tenth to the fourteenth inclusively. In Palcemon, two arthrobranchiae, one of which is very small, are attached to the arthrodial membrane of the external maxillipede, which has a very short and rudimentary epipodite. The second maxillipede bears a podobranchia divided into a small branchia and an oval epipoditic plate. In the first maxillipede the place of the podobranchia is occupied by a rounded bilobed lamella. In Crangon none of the maxillipedes bear gills. The epipodite of the first maxillipede is relatively much larger and triangular; that of the second is tongue- shaped and almost vesicular; that of the third is a mere rounded process. I can find only one arthrobranchia on the ninth somite. The Branchial formula o/Palaemon. Somites and their appendages. VII. . . VIIIIX. .. . X. . XI... XII. . XIII... XIV... Podobranchia?. • 0(ep) . 1 , 0 0 . 0 . 0 0 0 (ep) Arthrc Anterior. 0 0 1 0 0 0 0 0 branchia?. Posterior. 0 0 1 0 0 0 0 0 Pleurobranchia?. 0 = 0 = 0 - = = = = 0(ep) 1 2 + ep 1 1 1 1 1 H-2ep+l + 1 + 5 = 8 + 2 ep. From the number of their pleurobranchiae the Caridomorpha cannot be regarded as a reduced modification of any of the Trichobranchiata, except the Penaeida? and Stenopus. But it is easy to derive them from a Stenopus-like primitive form by the reduction of the podobranchiae and the arthrobranchiae, and the conversion of the five posterior pleurobranchia? into gills of the lamellar type. In the Brachyura of Milne-Edwards the disposition of the branchial apparatuses well known to be definite and characteristic. In Cancer pagurus, for example, there are nine branchiae ; seven of these are pyramidal in form, and take a general direction from the base towards the apex of the branchial chamber, to the inner walls of which they are applied. The two posterior of these gills are pleurobranchia?, being attached respectively to the epimera of the eleventh and the twelfth somites. The fifth and fourth, the third and second, |