Agnatha Cyclostomata

Division - Agnatha (lack jaws)

Class - Cyclostomata

  • Class Cyclostomata, also known as Agnatha, is a group of jawless fish that belong to the division Agnatha.
  • They are primitive vertebrates and are characterised by the absence of jaws and paired fins. Cyclostomes include two extant orders: Myxini (hagfish) and Petromyzontida (lampreys).


General Characteristics:

  • Cyclostomes are the most primitive and simplest vertebrates.
  • They lack true jaws and paired fins.
  • Their body is elongated and eel-like in shape.
  • They have a cartilaginous endoskeleton, lacking bone.
  • The body is covered with a slimy, scaleless skin.



  • Cyclostomes respire primarily through gills, although some species also possess primitive lungs.
  • Lampreys have a single nostril that connects to a nasohypophyseal sac, which is used for respiration.


Feeding Habits:

  • Cyclostomes are parasitic or scavenging feeders.
  • Lampreys are parasitic and attach themselves to the bodies of fish with their sucker-like mouth.
  • Hagfish feed on dead or dying organisms by burrowing into the flesh and sucking out tissues.


Circulatory System:

  • They possess a two-chambered heart with a single atrium and a single ventricle.
  • Blood circulation is simple and lacks a true capillary network.



  • Cyclostomes excrete nitrogenous wastes in the form of ammonia.
  • They lack specialised excretory organs like kidneys and excrete waste through their skin and gills.



  • Cyclostomes are mostly dioecious (having separate sexes).
  • Fertilisation is external, and they exhibit indirect development.
  • Lampreys build nests and lay eggs in freshwater, while hagfish release eggs directly into the water.


Economic Importance:

  • Some species of lampreys are consumed as food in certain cultures.
  • Hagfish secrete a sticky slime when disturbed, which is used in various industries for making leather and other products.



Petromyzon (Lamprey) and Myxine (Hagfish).