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.

The exoskeleton consists principally of calcifications forming dorsal and ventral shields which cover the head and abdominal region; the dorsal shield is formed of a few plates firmly united, the ventral shield of a single plate. The shields are composed of three layers, the middle layer being traversed by canals belonging to the dermal sense organs which open to the exterior by a series of pores. The tail is sometimes covered by scales. The orbits are widely separated and laterally placed. Paired appendages are absent. These curious forms are found in beds of Upper Silurian and Lower Devonian age. One of the best known genera is Pteraspis.

Suborder (2). Osteostraci.

The exoskeleton as in the Heterostraci consists of shields and scales, the shields being divisible into three layers. The anterior part of the body is covered dorsally by a single large shield which differs from those of the Heterostraci in having the inner layer ossified. The middle layer contains canals for the passage of blood vessels, but the exoskeleton shows no impressions of dermal sense organs. The posterior part of the body is covered by large quadrangular scales. Paired appendages are absent, but median dorsal and caudal fins occur supported by scales, not fin-rays. Cephalaspis, the best known of these animals, occurs in beds of Lower Devonian age.

Suborder (3). Antiarcha.

The exoskeleton is formed of bony plates, the dorsal and ventral shields each consisting of several symmetrically arranged pieces. The tail may be covered with small scales or may be naked. The head is articulated with the trunk, and its angles are drawn out into a pair of segmented paddle-like appendages, covered with dermal plates. The orbits are close together. A dorsal fin and traces of mouth parts occur in Pterichthys, but the endoskeleton is unknown. The best known forms Pterichthys[26] and Asterolepis occur in beds of Lower Devonian age.

General account of the skeleton of

Marsipobranchii.

The Marsipobranchii are worm-like animals. The living forms include two families, the Myxinoidei (Hags)—genera Myxine and Bdellostoma—and the Petromyzontidae (Lampreys).

Three species of Petromyzon are known, P. fluviatilis, P. marinus and P. planeri. The larval forms were for a long time thought to belong to a separate genus and were called Ammocoetes.

The Myxinoids, although very highly specialised in their own way, are at distinctly a lower stage of development than the adult Lamprey, and come nearer to the larval Lamprey or Ammocoete.

Spinal column.

In Myxinoids and larval lampreys, the notochord is enclosed in a thick chordal sheath, in connection with which in the tail region there occur cartilaginous pieces forming neural arch elements. In the trunk region, however, no cartilage occurs in connection with the spinal column, the only cartilage present being that forming the radiale of the dorsal fin. On the other hand in most species of lamprey (Petromyzon) cartilaginous pieces forming imperfect neural arches (fig. 4, B, 13) are found lying in the tough skeletogenous layer dorsal to the notochord, and extending throughout the whole length of the trunk and tail. Two of these pieces, which are probably homologous with the neural plates (see p. 72) of Elasmobranchs, occur to each neuromere, or segment as determined by the spinal nerves. The dorsal and caudal fins are supported by paired cartilaginous radiale which are connected proximally with the skeletogenous layer.

The Skull.

In Myxinoids the cranium is a mere cartilaginous floor without side walls or roof, and the trabeculae[27] end without growing forwards into cornua. In Lampreys the trabeculae grow forwards and send up plates of cartilage which meet above (fig. 4, 7) and form side walls and a roof for part of the brain case. In Lampreys a labial suctorial apparatus is well developed, including a large ring-like piece of cartilage (fig. 4, 2) which supports the oral funnel and bears a large armament of horny teeth. In Myxinoids on the other hand the labial skeleton is small and consists merely of barbels round the mouth.

The olfactory organ of Myxinoids has a very curious skeleton. It is covered with a kind of grating of cartilage which is prolonged in front into a tube composed of a series of imperfect cartilaginous rings. In Lampreys the olfactory organ opens merely by a short membranous passage. In correlation with the small development of the labial suctorial apparatus in Myxinoids the lingual apparatus is very greatly developed. The tongue in Myxine has been said to 'dominate the whole body' (Parker). It is supported by a great median cartilaginous bar which when followed forwards first becomes bifid and still further forwards becomes four-cleft.

The horny teeth in Myxinoids are chiefly borne on the very large supralingual apparatus. They form a double series arranged in the form of an arch. In Myxine there are seven large teeth and nine small ones on each side. In Bdellostoma the teeth of the two rows are more equal in size. In Bdellostoma and Myxine it has been shown that imperfect calcified teeth occur below the horny teeth.

In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed, but not excessively so. It consists of a long median cartilaginous bar which ends in front with a semicircular piece of cartilage supporting the median part of the tongue.

In both Myxinoids and Lampreys there is a complicated branchial basket apparatus, but while in Myxinoids the basket apparatus is interbranchial, formed deep within the head near the hypoblastic lining of the throat, in Lampreys it is extra-branchial and formed outside the head cavities (fig. 4, 10). The two sides of the basket apparatus in Myxine are not symmetrical. In the interbranchial basket apparatus of Myxinoids the hyoid and first and second branchial arches can be recognised. Traces of the interbranchial skeleton of Myxinoids can be detected in Lampreys, and similarly in Myxinoids, there are indications of the extra-branchial skeleton of Petromyzon. The branchial basket in Lampreys forms at its posterior end a kind of cup which supports the pericardium (fig. 4, 11).

A remarkable Cyclostome named Palaeospondylus[28] has recently been described from the Scottish Old Red Sandstone. It differs however from all living Cyclostomes, in having a spinal column formed of distinct vertebrae with well-developed neural arches. The caudal fin is well developed and the dorsal radiale are forked as in lampreys. The skull is well calcified and the auditory capsules are specially large. The mouth is very similar to that of lampreys, being circular and without jaws; it is provided with barbels or cirri. There is no trace of limbs and the average length is only about 1-1½ inches.