Class Cyclostomata: Cyclostomata is the modified and degenerate offshoot of the primitive vertebrate stalk. Because of their round mouth, they are named Cyclostomata. They’re parasitic, normally feeding on fish during their adult phase. Morphologically, they resemble eels. They are only vertebrates without genuine jaws, subsequently referred to as Agnatha. Cyclostomata consists of hagfishes and lampreys.

• All living members of the class Cyclostomata are ectoparasites [ives on the outside of its host] on some fishes.
• They have an elongated body bearing 6-15 pairs of gill slits for respiration.
• Cyclostomes have a sucking and circular mouth without jaws.
• Their body is devoid of scales and paired fins.
• Cranium and vertebral column are cartilaginous.
• Circulation is of closed type.
• Cyclostomes are marine but migrate for spawning [release or deposit eggs] to fresh water.
• After spawning, within a few days, they die. Their larvae, after metamorphosis [transformation from an immature form to an adult form in two or more distinct stages. Example: Larvae → Tadpole → Frog], return to the ocean.
• Examples: Petromyzon (Lamprey) and Myxine (Hagfish).

Sub-Divisions of Cyclostomata

The cyclostomes are sub-divided into two major orders.

Petromyzontiformes

Lampreys or lamper eels belong to this order.

• They are found in both marine and freshwater.
• They have a ventral mouth with many horny teeth.
• The nostril is present dorsally.
• They possess a well-developed dorsal fin.
• The dorsal and ventral roots of spinal nerves are separate.
• The development is indirect.
• Eg., Petromyzon, Lampetra

Myxiniformes

Hagfishes represent this order.

• They are found exclusively in the marine environment.
• They have a terminal mouth with few teeth.
• They have no buccal cavity.
• The nostril is terminal.
• They possess 6-14 pairs of gill slits.
• The dorsal and ventral roots of the spinal nerves are not separate.
• Eggs are large and few in number.
• The dorsal fin is usually absent, or weak.
• Eg., Myxine, Paramyxine.

Follow on Facebook

By Team Learning Mantras