Courses Vertebrates Agnatha

Agnatha are jawless fish that are the oldest known vertebrates. Lampreys and hagfish belong to this class. Agnatha are jawless fish that are the oldest known vertebrates.

Lampreys and hagfish belong to this class.

Characteristics of Class Agnatha

  • Phylum: Chordata
  • Subphylum: Vertebrata
  • Digestive system: do not have an identifiable stomach
  • Body: jaws are absent, paired fins are generally absent. Lampreys and hagfish do have a tail and a caudal fin. Both have slimy skin without scales or plates. Some extinct agnathans had thick body plates. They have a cyclostomic (circular) toothed mouth (Figure 8), which helps them  bore into the side of a fish and suck the blood of their victim
Cyclostome (circular mouth) of a Sea Lamprey
Figure 8: Cyclostome (circular mouth) of a Sea Lamprey
  • Skeleton: the internal skeleton of the Agnatha is not bony but rather cartilaginous.The embryonic notochord persists in the adult.
  • Respiratory system: seven or more paired gill pouches are present
  • Reproduction: unisexual (lamprey) as well as hermaphroditic (hagfish)

Table showing difference between Lamprey and Hagfish

Features Lamprey Hagfish
Scientific name Petromyzon (Figure 9) Myxine (Figure 10)
Body Stout, size may reach up to 1 meter, presence of two dorsal fins, not very slimy Feeble, smaller in size, no or single fin present, slimy so called “slimy eel”
Habitat Mostly marine; a few fresh water forms Exclusively marine
Habit Both parasitic and non-parasitic species Only parasitic species
Eyes Functional Degenerate
Mouth Ventral Terminal
Salivary gland Present and secrete an anticoagulant Absent
Gills Seven pairs of external gill slits One pair of external gill slits
 Brain  Well developed; has ten pairs of cranial nerve  Not well developed; eight pairs of cranial nerve
Nutrition Sucks out blood of host fishes in feeding Primarily scavenger and feeds mostly upon dead fishes in feeding
Reproduction Unisexual; breeds marine form migrates to fresh water river and stream for spawning (anadromous) Hermaphrodite; breeds as well as spawning on sea floor
Development Indirect with ammocoete (larva), metamorphosis present Direct without larva, metamorphosis absent
A Sea Lamprey (Agnatha)
Figure 9: A Sea Lamprey (Petromyzon marinus)

A Hagfish protuding from sponge (Source: NOAA)
Figure 10. A Hagfish protuding from a sponge (Source: NOAA)
In older classifications, both lamprey and hagfish were grouped together as Cyclostomata due to the presence of round/circular mouth.

Later, it was found that lampreys have more similarities with jawed forms (gnathostomes), such as, presence of lateral eyes, regulation of heartbeat etc.

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