Division-I Agnatha (Jawless Vertebrates)
Classification of Animal Chordates of Class 11
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.
- 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
Differences between Lamprey and Hag fish
|Marine as well as fresh water.||Exclusively marine.|
|Either external parasite or non-parasite.||Behaves as internal parasite.|
|Rasp away flesh & suck out blood of host fish.||Primarily scavengers.|
|Anadromous.||Spawn on ocean floor.|
|Salivary gland secretes anticoagulant.||Salivary glands absent.|
|10 pairs cranial nerves.||8 pairs cranial nerves.|
|Two semicircular canals.||One semicircular canal.|
|Sexes separate.||Sexes united.|
|Indirect development.||Direct development.|
Points to Remember
Petromyzon though marine yet goes to fresh water for spawning i.e., anandromous.
Ammocoete larva hatches out in about 14-21 days.
Ammocoete is connecting link between Amphioxus and the cyclostomes.
Lingual Cartilage is also part of skeleton, lies in the tongue region and supports it.
Typhlosole is a fold of epithelium in the intestine. It prevents the rapid flow of food in the intestine and increases the absorptive surface area.