Cyclostomata: Characteristics, Classification, & Examples

Cyclostomata are group of parasitic vertebrates that includes the living jawless fish, morphologically similar to eels. They are popularly known as Agnatha, due to their jawless mouth. They belong to phylum chordata of kingdom Animalia. Cyclostomes are believed to have split off before the evolution of dentine and bone, hence making them jawless. However, many fossil agnathans, including conodonts found, possessed dentine and bones. 

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What are Cyclostomes?

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Cyclostomes are the members of class Cyclostomata. The meaning of cyclostomata is derived from two Greek words-“cyklos which means circular and stoma meaning mouth”. Thus, Joan Crockford-Beattie came up with the name ‘Cyclostomata’ since the members of this group had a round mouth. It is a vertebrate group of animals which includes the living jawless fishes, lampreys and hagfishes.

• Such animals possess horny epidermal structures that act like teeth and branchial arches, positioned internally unlike related jaw fishes.
• Cyclostomes are warm blooded vertebrates found exclusively in marine environments.
• The members of phylum cyclostomata are marine vertebrates but for spawning, they migrate to freshwater. 
• Spawning is the process of reproduction in cyclostomes through which they lay eggs to give rise to new organisms. 

Cyclostomata

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Origin of Cyclostomata

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All living organisms are classified and identified based on the form and function of their bodies. The existence of cyclostomata first appeared in lower ordovician period. Some features are more prone than others to cause wide-ranging alterations in body design.

There is also a function of time in this. As a result, once a given body design is established, it will shape the impacts of all subsequent design alterations simply by existing. In other words, features that emerged earlier are more likely to be fundamental than characteristics that emerged later.

• Cyclostomata are living agnathans that are rudimentary in many ways but specialised in others.
• The suctorial mouth is ventral and circular, hence the name Cyclostomata.
• These are modified versions of the primitive vertebrates that are believed to have arisen in the Devonian period. 

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Characteristics of Cyclostomata

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Cyclostomes are early vertebrates with no jaws. Hagfish and lampreys are among them. The body is eel-like in length with a compressed tail and trunk. Some of the notable characteristics of Cyclostomes are:

• Paired fins are lacking. The cartilaginous fin-rays support the median fin.
• The skin is soft and smooth with a slimy texture and less scaling.
• The trunk and tail include Z-shaped myomeres. The tongue is moved by the protractor and retractor muscles.
• The stomach is missing and the oesophagus connects to the intestine.
• There is an endoskeleton present.
• Skull is a primitive and simplistic design.
• The notochord is present throughout one's life.
• Around the notochord, neural arches represent the vertebrae.
• In sac-like pouches, there are five to sixteen pairs of gills.
• The heart is divided into two chambers. There is a sinus venous, but no conus arteriosus.
• Leukocytes and irregular nucleated erythrocytes are found in the blood.
• The brain is visible.
• There are ten pairs or fewer cranial nerves present.
• The nasal sac is solitary and located in the middle of the nose.
• There is a lateral line sense organ.
• A pair of mesonephric kidneys are part of the excretory system.
• The sexes are distinct.
• Gonad is without a gonoduct and is solitary.
• Direct development or a protracted larval stage are also possibilities.

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Cyclostomata Resembling Amphioxus 

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Below mentioned are the characteristics of Cyclostomata resembling the characteristics of Amphioxus:

• There are no jaws, exoskeleton, or paired fins.
• Continuous notochord but with an added sheath.
• Presence of myotomy, that is segmental musculature, but with minimal variation from head to tail.
• Straight ciliated food tract with little regional specialisation.
• The number of gill-slits is relatively large.
• Lamprey larvae with endostyle. 
• No products and gonads.

Amphioxus 

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Difference between Cyclostomata and Fish

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Mentioned below are the notable differences between Cyclostomata and Fish:

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Classification of Cyclostomata 

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There are two orders in the Cyclostomata class, namely, 

1. Petromyzontida 
2. Myxinoidea

Petromyzontia

The characteristics of Petromyzontida are:

• Lampreys are included in this category.
• The buccal funnel has suctorial teeth and is suctorial.
• In the buccal funnel, the mouth is present.
• The nasal sac is located on the back of the nose. There is no link between it and the pharynx.
• The eyes are useful.
• There are seven pairs of gill slits.
• There is a well-developed dorsal fin.
• The branchial basket is finished.
• The brain has reached its full potential.
• The pineal eye has a good amount of development.
• The ear canals are semicircular.

Lamprey

Myxinoidea

The characteristics of Myxinoidea are:

• Hag-fishes, sometimes known as slime eels, are among them.
• The buccal funnel is missing.
• A canal connects the nasal sac to the pharynx.
• Eyes are a relic of the past.
• The dorsal fin is either missing or very tiny.
• The branchial basket isn't as developed as it could be.
• The brain is a basic organ.
• The size of the pineal eye has shrunk.
• There is only one semicircular canal in the ear.
• All hag-fishes are found in the sea.

Myxinoidea

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Things to Remember

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• Cyclostomata are parasite fish that feed on them when they are adults. They have a morphology that is similar to that of eels. 
• They are the only extant vertebrates lacking genuine jaws, which is why they are termed Agnatha. Hagfishes and lampreys are examples of Cyclostomata.
• The marine lamprey breeds in freshwater, which has been known for a long time. It does not, however, indiscriminately penetrate all streams within its range.
• Cyclostomata means ‘round mouths’ in Greek. They can't close their mouths because they don't have a jaw, so they have to constantly cycle water through it.
• Cyclostomes have a sucking and round mouth without the presence of a jaw.