GENERAL CHARACTERISTICS OF CYCLOSTOMATA

Gr.,cyklos=circular, stoma=mouth

May be marine or fresh water in habitat.
Body is elongated and Eel like. Body is divisible into trunk and tail. Trunk and tail muscles are segmented into myotemes separated by myocomata.
Paired fins are absent. Median fins are supported by cartilagenous fin ray.
Skin is smooth, soft and slimy. Scales are absent.
Notochord is persistent through out the life. Vertebrae are represented by imperfect neural arches (arcualia) present surrounding the notochord.
Pharyngeal gill-slits are found in which about 6-15 paires of gills are present.

Mouth without jaws, so grouped under Agnatha. Mouth is ventral, suctorial and is circular. Mouth is surrounded by tentacles.
Digestive system lacks stomach. Esophagus directly opens into intestine.
Excretion takes place by 2 mesonephric kindneys.
Heart is 2 chambered with 1 auricle and 1 ventricle. Conus arteriosus is found anteriorly. Hepatic portal system is well-develped. Blood contains R.B.C, leucocytes and are poikilotherms.
Nervous system includes a brain and a dorsal nerve cord. 8-10 cranial nerves are present.
A median nasal chamber is found with a single nostril. Auditory organ is with 1-2 semicircular rings.
Reproduction is sexual. Fertilization is external.
Development direct or by prolonged larval stage.
Dioecious animals.