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