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.

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).

Division-I Agnatha (Jawless Vertebrates)

Fig. Sea lamprey Petromyzon marinus

Division-I Agnatha (Jawless Vertebrates)

Fig. Myxine

Differences between Lamprey and Hag fish

Lamprey 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.

 

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