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 arthrobranchise in all essen- 

 tial 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 setse. 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 slender 

 anterior and superior process and the soft integument. Hence, the last 

 thoracic somite moves easily upon its predecessor, though, in strict- 

 ness, 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 may be termed a 

 pleurobranchia. The similarly attached filaments (i2plb and isplb) 

 represent reduced or rudimentary pleurobranchiae. 



We 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 

 arthrobranchise, fixed to the articular membrane ; and one pleuro- 

 branchia, 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. 



