The Cyclostomata are the modified and degenerate offshoot of the primitive vertebrate stalk. Due to their circular mouth, they are named Cyclostomata.
Characteristics of Cyclostomata
The body is round and elongated like an eel.
The paired fins are absent.
Median fins with cartilaginous fin rays.
No paired appendages.
The skin is soft and smooth, devoid of any scales.
Spleen is absent.
The exoskeleton is absent. The endoskeleton is cartilaginous with no bones.
The notochord is present throughout their lives.
The digestive system is devoid of any stomach.
The nostril is single and median.
The gills are five to sixteen in pairs.
The heart is two-chambered.
The brain is visible.
The lateral line acts as a sense organ.
About ten pairs of cranial nerves are present.
The sexes are separate. Some hagfish species are believed to be hermaphrodite.
A pair of mesonephric kidneys make up the excretory system.
Development may be direct or indirect.
Eg., Petromyzon and Maxine.
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