Division-I Agnatha (Jawless Vertebrates)

Jawless primitive fish-like vertebrates without true jaws  and paired limbs.

It is divided into two classes:

Ostracodermi (Extinct armoured fish)

• Body with heavy armour or large bony plates on the head and of small bony scales elsewhere; Palaeozoic, world’s first vertebrates e.g. Cephalaspis.

Cyclostomata

• Skin soft and scaleless.
• Mouth without jaws and remains permanently open.
• Mouth circular and suctorial.
• Median fins with cartilaginous rays, but no paired appendages.
• Cartilaginous endoskeleton.
• Digestive system lacks stomach. Intestine with a fold, typhlosole.
• Gill slits 5 to 16 pairs.
• Heart 2-chambered; poikilothermous.
• Dorsal nerve cord; 8 to 10 pairs of cranial nerves.
• Single median olfactory sac and nostril.
• Auditory organ with 1 or 2 semicircular canals.
• Sexes separate or united; fertilization external; development direct or with prolonged larval stage (larva is ammocoete e.g., lamprey). e.g., Lamprey (Petromyzon), Hag fish (Myxine).

Fig. Sea lamprey Petromyzon marinus

Fig. Myxine

Differences between Lamprey and Hag fish