Animals

Jawless Fish

by Marcia Malory

The jawless fish that are alive today are the hagfish (Hyperotreti or Myxini) and the lampreys (Hyperoartia).

Jawless fish are sometimes given the name Agnatha, which is Ancient Greek for "no jaw".

Both hagfish and lampreys are craniates - chordates with skulls.

However, while lampreys are vertebrates, hagfish are not.

Pacific hagfish.Hagfish have skulls, but they do not have spinal columns (backbones).

Every chordate has a notochord, a flexible supporting rod along the inside of its body, during some part of its lifecycle. In vertebrates, the notochord is usually replaced by a spinal column before the animal hatches or is born.

The hagfish keeps its notochord throughout its life. It never develops a backbone.

A lamprey also retains its notochord through adulthood. However, it also has small arcualia (pairs of cartilage) above the notochord.

Physical Characteristics

Jawless fish have long bodies and look like eels.

They have tails.

Their skeletons are made of cartilage, not bone.

They have no scales.

Lampreys and hagfish have glands in their skin that excrete slime.

They have no stomachs. The digestive tract of a jawless fish consists of its mouth, throat, intestine and anus.

Jawless fish are ectothermic (cold-blooded).

Fins

Unlike jawed fish, jawless fish have no paired fins.

Fish with jaws have a pair of  pectoral fins and a pair of pelvic fins.

There is one pectoral fin on each side of the fish's body.

The pelvic fins, which are also known as ventral fins, can be found below the pectoral fins, toward the abdomen.

In tetrapods, pectoral fins have become front limbs and pelvic fins have become hind limbs.

Although jawless fish do not have paired fins, they do have caudal fins (tail fins).

Lampreys also have two dorsal fins (fins on their backs).

Reproduction

Jawless fish practice external fertilization and are oviparous - the young develop in eggs that are outside the parent's body.

Feeding

Both hagfish and lampreys have round mouths that act like suckers. Jawless fish are sometimes known as cyclostomes, which is Ancient Greek for "circle mouth."

Jawless fish have very sharp teeth.

Because they do not have jaws, they cannot move their teeth up and down.

Hagfish feed by shredding the bodies of dead or injured animals.

A hagfish will sometimes enter the body of its prey via the prey's mouth, anus or gills and then feed on its prey from inside.

Sometimes, a hagfish will tie itself into a knot in order to give itself leverage when it is tearing off the flesh off of its prey.

Lampreys are parasites. A lamprey will use its teeth to grab onto the flesh of an animal and then suck out the animal's blood and its other bodily fluids. When it feeds, a lamprey will inject a fluid that prevents blood from clotting into its host.