Cyclostomes’ are jawless vertebrates. They are primitive vertebrates
. The cyclostomes are marine or fresh-water vertebrates.
It includes the lampreys and hag fishes.
- The body is long, eel like. It shows trunk and compressed tail.
- Paired fins are absent. Median fin is supported by cartilaginous fin-rays.
- The skin Is soft and smooth. It is slimy. It as scale less.
- Z shaped myomeres are present in the trunk and tail Protractor and retractor muscles move the tongue.
- In this group a true coelome is seen.
- These vertebrates will not have jaws, hence called Agnatha.
- The mouth is circular. It works like a sucker. It is surrounded by tentacles.
- Tongue bears teeth.
- Stomach is absent and oesophagus leads into the intestine.
- Endoskeleton is present.
- Skull is simple and primitive.
- Notochord presists throughout life.
- Vertebrae are represented by neural arches, around the notochord.
- Five to sixteen pairs of gills are present in sac like pouches
- The heart is two chambered. Sinus venousus is present, but conus arterlcsus is absent.
- Blood contains leucocytes and Irregular nucleated erythrocytes.
- Brain is seen.
- Ten pairs or less number of cranial nerves are present.
- Nasal sac is single and median.
- Lateral line sense organ is present.
- Excretory system includes a pair of mesonephric kidneys.
- Sexes are separate.
- Gonad is single and without a gonoduct.
- Development may be direct or with a long larval stage.