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Proceedings of the Royal Society of Edinburgh. [Sess. 
cf. Tait (1917, III, p. 81) — is that the problem of limb-design in relation to 
the environment is worthy of study in and for itself. By comparison with 
the limbs, say, of other reptant Crustacea and of insects, one might in 
this way arrive at wider conceptions. 
The Coxo-bctsal Articulation . — The most interesting articulation in the 
isopod limb is (of course) the coxo-basal. The arrangements pertaining to 
this joint may be described under four headings: (1) the articular part 
of the coxa, (2) the articular part of the basipodite, (3) the articular mem- 
brane, and (4) the muscles. 
It may be as well to offer first of all a few general remarks upon 
crustacean (or arthropodan) joints. The rigid links or segments between 
which the joints lie are not solid like bones ; they are open cylinders ; 
the articulating extremities are similarly portions of hollow cylinders. In 
movable crustacean joints the bearing surfaces are not oiled as are the 
diarthrodial joints of vertebrates ; they are not even inclosed within the 
articular membrane, though this is always continuous like the capsule of 
a diarthrodial bon}^ joint ; they lie as it were outside the “ capsule,” if one 
can make such a statement. These bearing surfaces, frequently two in 
number in each of the two segments contributing to the formation of a 
simple hinge joint, are stronger and more heavily built than the other 
parts of the extremities of the segments ; at the bearing surfaces alone 
do the articulating segments come into direct contact. The articular 
membrane, like the capsule of a diarthrodial joint, becomes thickened at 
parts (especially near the bearing surfaces) to form definite ligaments, 
these being invariably short. Generally speaking, in any two articulating 
segments of an appendage the proximal end of the distal segment is 
received into the dilated distal extremity of the proximal segment. One 
other difference between the arthropodan and the vertebrate joint may 
be mentioned, though this does not concern us just at present. The 
diarthrosis, or most movable joint, of the vertebrate is, so to speak, a 
terminal stage ; originating as a synarthrosis, it rarely reverts again to a 
synarthrodial condition. The crustacean joints, primitively movable, tend 
in many cases to develop into immovable joints — compare the union between 
basipodite and ischiopodite of the crab or between coxopodite and tergite 
of isopods, and also the very common concrescence of body segments in 
Crustacea. 
The articular part of the coxa is best described under two headings — 
the articular foramen and the bearing surface. In Glyptonotus the lateral 
projections of the coxae do not form ventrally hanging plates; the ventral 
surface of all the coxopodites is situated in the same horizontal plane as 
