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Subphylum : Vertebrata

In vertebrates, the notochord developed during the embryonic stage is replaced by a cartilaginous or bony vertebral column which forms a protection covering around the dorsal tubular nerve cord.

  • Head is well developed and sense organs are present. The brain is protected by a cranium.
  • They have a ventral muscular heart with one, two, three or four chambers.
  • Paired lateral appendages are present in the form of fins, flippers or limbs.
  • A post anal tail is present in most of the members.

The subphylum is divided into two divisions and many classes.

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  1. Division: AGNATHA

Division Agnatha consists of jawless vertebrates. Lateral appendages are also absent. This group is divided into 2 classes.

  1. Class Ostracodermi (extinct)
  2. Class Cyclostomata.

Class : Cyclostomata.

The term cyclo means circular, and stoma means opening or mouth. The members have a circular suctorial mouth (without jaws) in the ventral surface of the body. This is the characteristic feature of this group.

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Habitat: All members are aquatic (freshwater or marine). They live as ectoparasites on some fishes.

Examples: Petromyzon, Myxine

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Petromyzon (Lamprey) Myxine (Hagfish)

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Gill slits – 7 pairs

Body characteristics:

  • The body is slender, elongated and eel like.
  • It bears 6-15 pairs of gill slits for respiration.
  • The mouth is circular, suctorial and jaw-less.
  • Their body is devoid of scales and paired fins.

Notochord: Cranium and vertebral column (endoskeleton) are fibrous and cartilaginous. Notochord is persistent throughout the life.

Physiology: Circulation is of closed type.

Reproduction: Cyclostomes are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to the ocean.

 

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