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Show 1878.] AND DISTRIBUTION OF T H E CRAYFISHES. 763 The Branchial formula of Astacus fluviatili1 Somites p , Arthrobranchias. „, and their , rod.0: , • Pleuro-appendages. to*neh«. Anterior PoBterior. branchiae. VII 0(ep) 0 0 0 = 0 (ep) VIII 1 1 0 0 =2 IX 1 1 1 0 =3 X 1 1 1 o 3 XI 1 1 1 0 =3 XII 1 1 ] r = 3-f-r XIII I l l r = 3 + r XIV 0 0 0 1 =1 6 + ep+6 +5 + l+2r=184-ep + 2r " ep" here signifies a podobranchia which has lost its branchial filaments and become completely metamorphosed into an epipodite, while r indicates that a rudiment of a branchia exists. It will be observed that, in this species of Crayfish, no somite possesses its hypothetically full complement of branchiae except XII. and XIII.; and even in them tbe pleurobranchiae are rudimentary. The representatives of eleven possible branchiae are altogether wanting. 2. The Branchiee of Cambarus. The principal distinction between this genus and Astacus, as it was established by Erichson, lies in the absence of the single pleuro-branchia of the latter, and the consequent reduction of the number of the branchiae to seventeen on each side. In his elaborate monograph of the genus, Dr. Hagen observes, "But there is also another difference, not before noticed2. In Astacus each pair of gills, except the single one on the fifth set of legs, has a broad deeply-folded membrane, closely fixed behind the most external gill-lobe. In Cambarus, this membrane is always wanting in tbe gills of the fourth pair of legs, but exists, as in Astacus, in all the others. " In the true Astacus, all the gills with a folded membrane behind have a basal external bundle of shorter but broader and irregularly placed gill-tubes ; these are never to be found in Cambarus." In a species of Cambarus from Guatemala, of which a number of specimens have been presented to the British Museum by Mr. Salvin3, I find Dr. Hagen's first remark fully borne out. The last 1 In this, as in all other cases, it is to be understood that the branchial formula gives the branchiae of only one side of the body, and that the total number of branchia? is therefore double that given in the formula. 2 Dr. Hagen appears to have overlooked De Haan's definition of the distinctive characters of the American Crayfishes known to him :-• " Branchiae 17 ; nulla supra pedes quintos; externes supra quartos tantum e tubulis liberis, externa; supra sequentes infra e tubulis, supra e laminis tuber-culatis compositae " (Fauna Japonica, Crustacea, p. 164). 3 Mr. Salvin informs me that they were obtained in a river near Coban, in Vera'Paz, at an elevation of about 4300 feet above the sea. |
