longitudinal bands, especially developed toward the dorsal surface
of the body to the right and left of the axial skeleton. Locomotion
was produced by lashing the tail right and left, as still in fish.
There is improvement in all these organs, except perhaps the
reproductive, but nothing very new or striking. The great
improvement from this time on was not to be sought in the vegetative
organs, or even directly to any great extent in muscles.

The new and characteristic organ was not the vertebral column, or
series of vertebræ, or backbone, from which the kingdom has derived
its name. This was a later production. The primitive skeleton was
the notochord, still appearing in the embryos of all vertebrates and
persisting throughout life in fish. This is an elastic rod of
cartilage, lying just beneath the spinal marrow or nerve-cord, which
runs backward from the brain. The nerve-centres are therefore here
all dorsal, and the notochord or skeleton lies between these and the
digestive or alimentary canal. The skeleton of the clam or snail is
purely protective and a hindrance to locomotion. That of the insect
is almost purely locomotive, but external, that of the vertebrate
purely locomotive and internal. It does not lie outside even of the
nervous system, although this system especially required, and was
worthy of, protection. It does not protect even the brain; the skull
of vertebrates is an after-thought. It is almost the deepest seated
of all organs. But lying in the central axis of the body it
furnishes the very best possible attachment for muscles. Around this
primitive notochord was a layer of connectile tissue which later
gave rise to the vertebræ forming our backbone.

[Illustration: 10. CROSS-SECTION OF AXIAL SKELETON OF PETROMYZON.
HERTWIG, FROM HIEDERSHEIM.