81 
1916-17.] Experiments and Observations on Crustacea. 
The muscles at the joint in this region are strongest ; the skeletal parts 
are also strongest.* 
On change from water to land the question of gravity in its relation 
to the design of the limbs comes immediately to the forefront, for the 
bending moment at the proximal part of the limb is then greatly and 
permanently increased. In Peracarida with tanaidacean limb-taxis the 
transverse distance between the proximal part of any pair of limbs is 
slight, and on general grounds one might say that there is no available 
room for transversely arranged muscles to counteract the bending moment 
at the most proximal flexure. I shall not follow the problem into its 
details, for it is an involved one, and in any case cannot be settled simply 
on a priori reasoning. It is along these lines, however, that we may hope 
to explain the eventual success of isopods and amphipods in accommodating 
themselves to the conditions associated with a reptant life on land. 
Rotation of the Plane of Flexure of the Limb. 
In a developing mammal the limbs, originally straight outgrowths 
from the side of the body, by alternate flexions in one plane become bent 
into the position of the limbs of a tortoise. In this bending the “ pre-axial 
border ” remains anterior. Then the upper or humeral segment of the 
anterior extremity is rotated horizontally backwards from the shoulder 
through a right angle, while the corresponding segment of the posterior 
extremity is rotated to the same extent forwards from the hip. The 
consequence is that the pre-axial border of the fore-limb is lateral and 
the elbow points backwards, whereas the post-axial border of the hind-limb 
is lateral and the knee points forwards. 
In the reptant Peracarida we are evidently confronted with a problem 
of analogous nature. As in the mammal, the site of rotation lies at the 
proximal part of the limb. In the bony limb the presence of a spheroidal 
joint may be essentially bound up with the possibility of this rotation 
(the retention of permanent “ rotatibility ” along with unimpared action 
of the proximo-distally arranged muscles suggests this). The working 
out of the corresponding problem as it affects the arthropod limb would 
not only have a systematic bearing but would form an interesting contri- 
bution to the study of analogy, a branch of inquiry instinctive to the 
* This is independent of the question of inertia , which brings about reduction of mass, 
and consequently of muscle and supporting material, as one proceeds towards the distal 
extremity of the limbs of an animal designed for speed. The shifting of the muscles to the 
proximal part of the limb of an antelope and the provision of long tendons in the distal 
part come under this category. 
XXXVII. 
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