General characteristics and classification of cyclostomes upto class

 

General characteristics and classification of cyclostomes upto class

 

The Cyclostomata are a modified and degenerate offshoot of the primitive vertebrate lineage. They are named Cyclostomata due to their circular mouths. These creatures are parasitic, typically feeding on fish during their adult stage, and morphologically resemble eels. They are the only living vertebrates without true jaws, which is why they are also known as Agnatha. The Cyclostomata group includes hagfishes and lampreys.

Characteristics of Cyclostomata

1. The body is round and elongated, resembling an eel.

2. Paired fins are absent.

3. Median fins have cartilaginous fin rays.

4. No paired appendages.

5. The skin is soft, smooth, and scale-free.

6. Spleen is absent.

7. The exoskeleton is absent; the endoskeleton is cartilaginous with no bones.

8. The notochord is present throughout their lives.

9. The digestive system lacks a stomach.

10. There is a single, median nostril.

11. They have five to sixteen pairs of gills.

12. The heart is two-chambered.

13. The brain is visible.

14. The lateral line functions as a sense organ.

15. About ten pairs of cranial nerves are present.

16. The sexes are separate, although some hagfish species may be hermaphroditic.

17. The excretory system consists of a pair of mesonephric kidneys.

18. Development can be direct or indirect.

19. Examples include Petromyzon and Maxine.

 

 Classification of Cyclostomata

The class Cyclostomata is divided into two orders:

1. Petromyzonia

2. Myxinoidea

 Order 1: Petromyzonia

1. Includes lampreys.

2. The buccal funnel is suctorial with horny teeth.

3. The mouth is located within the buccal funnel.

4. The nasal sac is dorsal and does not connect with the pharynx.

5. Eyes are functional.

6. Seven pairs of gill slits are present.

7. A well-developed dorsal fin is present.

8. The branchial basket is complete.

9. The brain is well-developed.

10. The pineal eye is well-developed.

11. The ear has two semicircular canals.

Example: Petromyzon (Sea lamprey), Lampetra fluviatilis.

 

Order 2: Myxinoidea

1. Includes hagfish or slime eels.

2. The buccal funnel is absent.

3. The nasal sac opens into the pharynx through a canal.

4. Eyes are vestigial.

5. The dorsal fin is absent or very small.

6. The branchial basket is poorly developed.

7. The brain is primitive.

8. The pineal eye is reduced.

9. The ear has only one semicircular canal.

10. Hagfish are exclusively marine.

Example: Myxine glutinosa (Hagfish or slime eel), Eptatretus (Bdellostoma).