or annelid, but immensely increased in number, modified, and improved in certain very important particulars.  The muscles in simplest forms are composed of heavy 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 vertebrae, 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 vertebrae forming our backbone.

[Illustration:  10.  CROSS-SECTION OF AXIAL SKELETON OF PETROMYZON.  HERTWIG, FROM HIEDERSHEIM. SS, skeletogenous layer; Ob, Ub, dorsal and ventral processes of SS; C, notochord; Cs, sheath of notochord; Ee, elastic external layer of sheath; F, fatty tissue; M, spinal marrow; P, sheath of M.]

The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal marrow running down the back.  The brain of all except the very lowest vertebrates consists of four portions:  1.  The cerebrum, or cerebral lobes, or simply “forebrain,” the seat of consciousness, thought, and will, and from which no nerves proceed.  Whether the primitive vertebrate had any cerebrum is still uncertain. 2.  The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects.  Its anterior portion appears from embryology to be very primitive. 3.  The small brain, or cerebellum, which in all higher forms is the centre for co-ordination of the motions of the body. 4.  The medulla, which controls especially the internal organs.  The spinal marrow, or that portion of the nervous system which lies outside of the head, is at the same time a great nerve-trunk and a centre for reflex action of the muscles of the body.  But the development of these distinct portions and the division of labor between them must have been a long and gradual process.