Biology Sikshya

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​    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.

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• 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.