250 
Proceedings of the Royal Society of Edinburgh. [Sess. 
progression. The movement at the mero-carpal articulation supplements 
the amount of this antero-posterior swing so far as the distal portion of 
the limb is concerned. 
The existence of the extra movements at the coxo-basal and mero- 
carpal articulations serves to establish an increased similarity between the 
flexural arrangements of the isopodan and those of the mammalian limb. 
In the human arm, for example, not only may rotation of the humerus 
about its own axis occur, but also flexion of the bone in three separate 
planes at right angles to each other. The mero-carpal flexion of the isopod 
limb corresponds in relative direction, and in serial order in the full flexion- 
complex, to the latero-medial flexion between wrist and forearm. When 
one compares in detail the full flexion-complex of the isopod limb with 
that of the limb of a shore-crab, the very contrast in design between the 
two crustacean types serves if anything to heighten the similarity between 
the flexural arrangements of the isopodan and of the mammalian limb.* 
To save space the comparison with the crab may be presented in the form 
of a table, the limb in each case being supposed to lie in the transverse 
plane and to be arranged in the ordinary position for walking. 
Name of 
Articulation. 
Coxo-somitic . 
Coxo-basal 
Basi-ischial . 
Ischio-meral . 
Mero-carpal . 
Carpo-propodal 
Propodo-dactylic 
Direction of Axis of Joint. 
Variety of Joint. 
Crab. 
Isopod. 
Crab. 
Isopod. 
Vertical 
Simple hinge 
Segmental fusion 
(patent door) 
/"Movement about 
Antero- 
| 3 axes : rota- 
! Simple hinge 
Analogous to 
posterior 
1 tion about long 
| (patent door) 
“ spheroidal ” 
l axis of basip. 
J 
Antero-posterior 
Segmental fusion 
Simple hinge 
Vertical 
55 
Simple hinge 
55 
Antero- 
Vertical 
55 
Simple hinge 
posterior 
(patent door) 
Transverse 
An tero-posterior 
Simple hinge 
Simple hinge 
(patent door) 
Antero- 
55 
Simple hinge 
55 
posterior 
To summarise : — All the articulations in the brachyuran limb are 
simple hinges, and have only one degree of freedom of movement ; the 
flexion-complex in the principal plane (the uniplanar flexion-complex) is 
* It should be noted that the isopodan limb during progression has a different align- 
ment from the mammalian. One can demonstrate the mode of movement of the isopodan 
limb by moving sideways with one’s lower limbs flexed. 
