80 
Proceedings of the Royal Society of Edinburgh. [Sess. 
plainly shown, the taxis suggesting a simple inversion of the oniscoidean 
taxis. 
Let us take the argument from analogy. Vertebrates have only two 
pairs of limbs between which to differentiate ; the same kind of distinc- 
tion occurs in them so soon as they are used for crawling. 
Lastly, the functional argument. Suppose that the whole seven limbs 
of a reptant peracaridan were like the anterior three or four ; the animal 
would be adapted for forward progression but not for halting dead or 
for holding on against a current of water directed from behind. Generally 
speaking, one should also bear in mind the important fact (explanatory 
of many things), that an animal creeping in water derives little attachment 
to the under-surface from gravity. 
On all these counts it seems certain that the separation is an essential 
concomitant of a successful reptant life — at least in cases where the flexion- 
complex is uniplanar. 
Several subsidiary questions remain for discussion. Is there any 
particular reason why the separation should commonly occur between the 
fourth and fifth walking limb ? why should the line of division vary in 
position (in some isopods it occurs between the third and fourth pair 
of limbs) ? and what leads to obliteration of the distinction in the 
Oniscoidea ? While I intend to recur to these matters, I should here 
say that it is more easy to ask such questions than to provide answers. 
(The reader might perhaps refer to the immediately preceding paper of 
this series.) The most interesting question of all, and the greatest puzzle, 
is the presumable development of new neuro-muscular mechanisms in the 
reptant limb. 
Land Adaptation . — The Mysidacea, the Cumacea, and the Tanaidacea 
are exclusively aquatic. Only among the Isopoda and the Amphipoda 
do we meet with land forms. In such cases the animals never have 
tanaidacean limb-taxis. The taxis which has proved suitable for pro- 
gression on land is either isopodan or amphipodan. 
In considering the design of any limb, reptant or natant, one might 
call attention to a mechanical principle known as “ bending moment.” In 
a branch of a tree breakage from wind or weight is most apt to occur at 
the junction with the trunk, and here the strength has to be greatest. 
The wood is hardest here ; the branch also tapers as it is followed 
outwards, not simply in accordance with the provision of nutritional tubes 
for the leaves, but also for statical reasons. So in the limbs of an animal 
(even in a land animal they are never maintained all the time in a vertical 
position) the greatest bending moment is at the junction with the body. 
