120 COMPARATIVE ANATOMY 



(third of the thoracic, eighth of the whole series of appendages), 

 and we will accordingly examine it to see what light it throws 

 on the structure of the thoracic limbs behind it. When the 

 limb is removed from the body, by carefully cutting through 

 the arthrodial membrane at the base of its proximal joint, we 

 see (fig. 27, VIII) that it consists of two stout basal joints or 

 segments which we have no difficulty in recognising as the 

 coxopodite and the basopodite. The coxopodite is produced 

 externally into a stout process which bears a large plume-like 

 appendage, the gill or branchia, sometimes distinguished as 

 the epipodite : the details of its structure may be neglected 

 for the moment. The basipodite bears on its outer margin a 

 relatively short and slender branch, the exopodite, which is 

 very like the exopodite of a typical abdominal limb, consisting, 

 as it does, of a longish proximal joint bearing a jointed terminal 

 filament. The endopodite is borne at the extremity of the 

 basipodite, is stout and leg-like, and consists of five joints, 

 named from the proximal to the distal end as follows : ischio- 

 podite, meropodite, carpopodite, propodite, dactylopodite. 

 (In the crayfish the basipodite of the third maxilliped is 

 immovably fused with the ischiopodite.) If now we place the 

 seventh thoracic leg (twelfth of the whole series) of the same 

 side alongside the third maxilliped it is obvious that it is 

 made up of the same parts, minus the exopodite ; it consists, in 

 fact, of the protopodite with its two joints coxopodite and basi- 

 podite (the former bearing a gill or epipodite), and the endo- 

 podite composed of five joints, named as above (fig. 26, XII). 

 The last thoracic limb is similar to the seventh, but has no 

 epipodite. 



The fifth and sixth thoracic limbs (tenth and eleventh of the 

 whole series) are chelate that is to say, the penultimate joint 

 or propodite is somewhat expanded and its inner side is pro- 

 duced distally into a finger-like process upon which the terminal 

 joint or dactylopodite shuts down to form a pincer. The great 

 chelae have exactly the same structure, but the joints are larger 

 and stouter, the propodite is much expanded, and its cavity is 

 enlarged to give room for the powerful muscles which work the 

 dactylopodite. In it also, as in the third maxilliped, the 

 ischiopodite is immovably fused to the basipodite. 



Passing forward to the second maxillipede we find (fig. 27, 

 VII) that it has much the same structure as the third, except 



