1893.) Joint Formation Among the Invertebrata. 91 
valleys between the folds, this being aided by some of the cir- 
cular fibers which pass through the longitudinal sheath, and 
find their attachment to the bases of the valleys. 
Starting from this point, and supposing the regularity 
of the folds to have become established from pre-existing 
irregular folds by the regularity and stress of muscular action, 
we can conceive that when deposits of calcareous matter took 
place, rings similarly formed by a folding of a soft skin 
would receive that deposit at the most prominent portion of 
this fold, the convex face and not in the protected valleys, as 
there would be more friction or pressure from external 
causes, and no deposits would take place in the valleys them- 
selves because they would not be subject to external friction, 
and their continual flexion would prevent any such deposits. 
Should such a deposit take place in the valleys, there would be 
a stiffening of the whole surface which would defeat motion. 
In fact, in the leech, the cuticle is already much thicker on 
the crests of the folds than in the valleys. 
In the more primitive crustacea, we find the animal made up 
of ‘rings extending over the whole length of the body, similar 
to the rings of the leech, save (fig. 2) that there is but one ring 
i ‘to one somite, and instead of a per- 
pendicular valley between the folds, 
this valley has an inward and a 
forward direction, allowing the ante- 
s rior edge of a caudad ring to fit into 
the posterior edge of a cephalad 
ring. 
In the higher crustacea, several of 
the anterior rings have coalesced, 
and form a solid shield which is 
known as the carapace. This has 
no doubt arisen by the lessening of 
the action between the anterior rings 
oe T . : became the more active propelling 
kepama becca aed organ. As the action ceased forward 
showing the action of the mus- the valleys came to rest, and became 
cles. ‘exposed to friction and pressure, and 
