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Show 756 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, to the diminution of the number of the branchiae in Cambarus and Astacus, and to an important difference in the structure of those of Engceus; and Hagen has pointed out some important peculiarities of these organs in Cambarus; while the remarkable fact, that the appendages of the first somite of the abdomen are absent in many of the Crayfishes of the southern hemisphere, has been duly noted by Erichson and several other zoologists. Having recently had occasion to make a careful reexamination of the structure of Astacus jiuviatilis, I found two minute filaments attached to the epimera of the penultimate and antepenultimate thoracic somites. The structure and the position of those filaments led me to suspect that they must be rudimentary branchiae l; and as the Australian Crayfishes appeared to me to be, on the whole, less specialized forms than the European species, I thought that I should probably find in them fully-formed functional branchiae occupying the place of these rudiments. Through the kindness of m y friend and former pupil, Mr. J. Wood-Mason, a specimen of "Astacoides" franklini was placed at m y disposal; and on examination, I not only found the functional branchiae I sought, but discovered a number of other interesting differences between the respiratory organs of this Crayfish and those of Astacus. Following up the line of inquiry thus suggested, I have examined examples of all the chief forms of Crayfishes at present known, with the result of establishing some remarkable parallel relations between the morphology and the distribution of these animals. In order to make these points clear, I must premise a fuller and more precise description of the branchial apparatus of the common Crayfish than has yet been given, in order that it may serve as a standard of comparison for the branchiae of the other Crayfishes. II. THE MODIFICATIONS OF THE BRANCHIAE IN THE CRAYFISHES. The Branchiae of Astacus fluviatilis. "When the branchiostegite of a Crayfish is removed, seven branchiae are seen, running from the base towards the apex of the branchial cavity, parallel with one another, and disposed in curved lines, which are concave forwards and convex backwards. The length of the branchiae gradually increases from the first to the sixth; the seventh ascends as high as the sixth, but is rather shorter, in consequence of the attachment of its base lying at a higher level. In each of the six anterior branchiae, a basal portion, a stem, an expanded lamina, and an apical plume may be distinguished. The basal portion (fig. I, i, B ) is broad, with a convex posterior and inferior free edge, beset with long setse ; and it is articulated by its 1 I have met with no allusion to these structures, unless the following passage in Brandt's and Ratzeburg's description of the Crayfish (Med.-Zoologie, ii. p. 61) refers to them :-" Ueber jeder der obersten Kiemen der beiden mittlern Fusspaare steht etwa 1"' entfernt ein kleiner fadenformiger, unten breiterer, bartelahnlieher Theil." I do not quite see the applicability of " unten breiterer," unless " unten " applies to the attached ends of the filaments; but in other respe ts the description fits the rudimentary branchiae very well. |