In vertebrates, the notochord developed during the embryonic stage is replaced by a cartilaginous or bony vertebral column which forms a protection covering around the dorsal tubular nerve cord.
- Head is well developed and sense organs are present. The brain is protected by a cranium.
- They have a ventral muscular heart with one, two, three or four chambers.
- Paired lateral appendages are present in the form of fins, flippers or limbs.
- A post anal tail is present in most of the members.
The subphylum is divided into two divisions and many classes.
Division Agnatha consists of jawless vertebrates. Lateral appendages are also absent. This group is divided into 2 classes.
- Class Ostracodermi (extinct)
- Class Cyclostomata.
Class : Cyclostomata.
The term cyclo means circular, and stoma means opening or mouth. The members have a circular suctorial mouth (without jaws) in the ventral surface of the body. This is the characteristic feature of this group.
Habitat: All members are aquatic (freshwater or marine). They live as ecto–parasites on some fishes.
Examples: Petromyzon, Myxine
Petromyzon (Lamprey) Myxine (Hagfish)
Gill slits – 7 pairs
- The body is slender, elongated and eel like.
- It bears 6-15 pairs of gill slits for respiration.
- The mouth is circular, suctorial and jaw-less.
- Their body is devoid of scales and paired fins.
Notochord: Cranium and vertebral column (endoskeleton) are fibrous and cartilaginous. Notochord is persistent throughout the life.
Physiology: Circulation is of closed type.
Reproduction: Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.