OCR Text |
Show 762 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, of this filament is sometimes vertical, but more frequently horizontal. The anterior filament is sometimes a mere papilla; it is attached to the margins of a small foramen which occupies a similar position in the antepenultimate epimeron-namely, close to the anterior edge and just below the transverse depression. These are two rudimentary gills, of the same order as that next to be described. The seventh, and most posterior branchia of those which become visible when the brachiostegite is removed (fig. 3, uplb), has yet to be considered. It resembles one of the arthrobranchiae in all essential characters, but it is not attached to the arthrodial membrane; on the contrary, the base of its stem is fixed to the margins of a circular aperture situated close to the edge of a peculiar shield-shaped plate, the posterior and outer surface of which is covered with strong setae. Immediately behind and below the attachment of the gill there is an oval space, occupied by a soft and flexible portion of the cuticle, like a tympanic membrane. By its lower margin this plate furnishes an articular surface to the outer condyle of the coxopodite of the last thoracic limb, while its anterior and upper angle, bending sharply upwards, passes into a curved prolongation, which extends upwards and backwards in the soft integument of the flank, and articulates with a slender process of somewhat similar form sent forward from the first abdominal somite. Internally this shield-shaped branchiferous plate is continuous with the sternum of the last thoracic somite. It is obvious that this plate, with its anterior process, represents the epimeron of the last thoracic somite, which is thus adherent to the penultimate somite only by the sleuder anterior and superior process and the soft integument. Hence, the last thoracic somite moves easily upon its predecessor, though, in strictness, the usual statement that the last thoracic somite in Astacus is " free" is not altogether exact. It follows from this determination of the nature of the shield-shaped plate, that the gill which it bears is attached to the epimeron, or side-wall, of the last thoracic somite; and it m a y be termed a pleurobranchia. The similarly attached filaments (12plb and izplb) represent reduced or rudimentary pleurobranchiae. W e may suppose that the total number of branchiae which a thoracic somite can possess is eight, four on each side, namely:-one podobranchia, connected with the coxopodite of the appendage ; two arthrobranchiae, fixed to the articular membrane; and one pleurobranchia, attached to the epimeron. And if four places for branchiae are assigned to each somite, the extent to which the hypothetically complete scheme or formula is actually filled up will be readily seen, and the branchial arrangements of different Crayfishes will be easily compared. |