subphylum includes the well-known genus Amphioxus[23]. In Amphioxus the skeleton is very simple. It contains no trace of cartilage or bone and remains throughout life in a condition corresponding to a very early stage in Vertebrata. The skeleton of Amphioxus is partly hypoblastic, partly mesoblastic in origin.
(a) Hypoblastic skeleton.
The notochord (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord. It lies ventral to the nerve cord, and shows no trace of segmentation. It is chiefly made up of greatly vacuolated cells containing lymph, but near the dorsal and ventral surfaces the cells are less vacuolated. The notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath, and outside this comes the mesoblastic skeletogenous layer, which also surrounds the nerve cord.
The branchial skeleton. This consists of a series of chitinous elastic rods which strengthen the gill bars and are alternately forked and unforked ventrally. The forked rods are primary, and are U-shaped in section, the unforked rods are secondary, and are circular in section. All these rods are united at intervals by transverse rods.
(b) Mesoblastic skeleton.
The buccal skeleton. On each side of the mouth there is a curved bar resembling the notochord in structure. The bars are segmented, and each segment bears a smaller rod which supports a tentacle, the whole forming the buccal skeleton (fig. 3, 4).
The notochord is enclosed in a thick sheath of connective tissue continuous with a thinner sheath round the nerve cord. The sheaths of the notochord and nerve cord together form the skeletogenous layer, and prolongations of it form the myomeres or septa between the myotomes or segments of the great lateral muscles of the body.
The skeleton of each median fin consists of small cubical masses of a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving to strengthen the fins.
CHAPTER IV.
SUBPHYLUM D. VERTEBRATA.
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.
| 1. horny teeth. | 8. lateral distal mandibular. |
| 2. labial cartilage. | 9. lingual cartilage. |
| 3. anterior dorsal cartilage. | 10. branchial basket. |
| 4. posterior dorsal cartilage. | 11. cartilaginous cup supporting |
| 5. nasal capsule. | pericardium. |
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