The animals included in this great group all possess an internal axial skeleton forming the vertebral column or back-bone; and a dorsal spinal cord. The vertebral column is developed from the skeletogenous layer, which surrounds the spinal cord together with the notochord and its sheath; and in the great majority of cases the notochord becomes more or less modified and reduced in the adult. In some cases the notochord remains unmodified and the skeletogenous layer surrounding it is not segmented to form vertebrae, but in every case the neural arches which protect the spinal cord are segmented. The notochord never extends further forwards than the mid-brain.
All true vertebrates possess a cranium or skeletal box enclosing the brain.
(I.) Cyclostomata.
The mouth in living forms is suctorial and is not supported by jaws. In some fossil forms the character of the mouth is unknown.
Order I. Marsipobranchii[24].
In these animals limbs and limb girdles are always completely absent. They have no exoskeleton except horny teeth.
The endoskeleton, excluding the notochord, is entirely cartilaginous or membranous. The axial skeleton consists of a cartilaginous cranium without jaws, succeeded by a thick persistent notochord enveloped in a sheath. The notochord in living forms is unsegmented, but segmented cartilaginous neural arches are present in some cases. A complicated series of cartilaginous elements occurs in relation to the mouth, gills, and sense organs. The median fins are supported by cartilaginous pieces, the radiale. The order includes the Lampreys and Hags.
Order II. Ostracodermi[25].
The forms included in this group have long been extinct, being known only from beds of Upper Silurian and Lower Devonian age. They differ much from all other known animals. The exoskeleton is always greatly developed and includes (1) large bony plates covering the anterior region; (2) scales covering the posterior region. The plates are deeply marked by canals belonging to dermal sense organs. Jaws are unknown, and arches for the support of the appendicular skeleton are rudimentary or absent. The tail is heterocercal (see p. 60).
Suborder (1). Heterostraci.