Cyclostome

The living agnathans are the Cyclostomata (Greek: cyklos = round + stoma = mouth). They are basic in many ways, yet specialised in others. They originated in the Devonian as a modified and degenerate branch of the basic vertebrate stalk. They are parasitic or scavengers on fish when they are adults. They have a spherical body with a diphycercal or laterally compressed tail. On the surface, they resemble eels. The live jawless fishes, such as lampreys and hagfishes, belong to the Cyclostomata family of agnathans.

Both taxa feature jawless mouths with horny epidermal structures that serve as teeth and branchial arches that are positioned internally rather than externally as in jawed fishes. Cyclostomata means "round mouths" in Greek. Joan Crockford Beattie came up with the name. A long, thin body with no scales or fins, a circular jawless mouth with horny teeth, a cartilaginous skull, and a persistent notochord identify these fish.

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Cyclostomes

The suctorial mouth is round and ventral (hence, Cyclostomata). A muscular tongue with epidermal teeth rasps the flesh of fishes in the buccal cavity. They are the only vertebrates that do not have jaws. Different species have 6-14 pairs of internal gills. Gill chambers are circular pouches (hence, Marsipo Brachii). Though median fins are present and supported by cartilaginous fin rays, the epidermis is soft and devoid of scales, and paired appendages are absent. There is no prosthesis. The spinal column is primitive and the endoskeleton is cartilaginous with no bones. In each auditory organ, there are just one or two semicircular canals and a single middle nostril. The heart is protected by cartilage from the rear visceral arch. They don't have a spleen. Lampreys and hagfishes have Cyclostomata. Because they developed from distinct orders of ostracoderm, their two orders are not closely linked. 

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General Characters of Cyclostomes

• Long, rounded, and eel-like body.

• Fin rays are cartilaginous, however, there are no paired appendages on the median fins. Diphycercal tail.

• Skin that is soft and smooth, with unicellular mucous glands but no scales.

• Myotomes segment the trunk and tail muscles, which are separated by mycommata.

• Fibrous and cartilaginous endoskeleton. The notochord is present throughout one's life. Rudimentary vertebrae are represented by imperfect neural arches (arcualia) above the notochord.

• Jaws are missing (Group Agnatha).

• Ventral, suctorial, and a round mouth. The class name Cyclostomata (Gr., cyclos = round, stoma = mouth) comes from the circular mouth.

• The nostril is single and in the middle.

• Without a stomach, the digestive system works. Typhlosole is a fold in the intestine.

• Marsipo Brachii is another name for the class Marsipo Brachii, which has 5 to 16 pairs of gills in lateral sac-like pouches of the pharynx. 1 to 16 pairs of gill-slits

• A two-chambered heart with one auricle and one ventricle with an anterior conus arteriosus. In the gill area, there are many aortic arches. There is a hepatic portal system in place. Leukocytes and nucleated circular erythrocytes in blood. Variable body temperature (poikilothermic).

• Two metanephric kidneys connected to the urogenital papilla viaducts.

• With a differentiated brain, the dorsal nerve cord has 8 to 10 pairs of cranial nerves.

• There is only one olfactory sac in the middle.

• One or two semicircular canals make up the auditory organ.

• Sexes can be divided or unified. Single, big gonad without gonoduct.

• Fertilization from the outside. Director of development with a lengthy larval stage.

Possible External Relationships

The paraphyletic superclass Agnatha, which also includes numerous extinct armoured fishes known as ostracoderms, sometimes includes this taxon. The majority of ancient agnathans, such as galeaspids, thelodonts, and osteostracans, are more closely linked to gnathostomes (vertebrates with jaws) than to cyclostomes. Cyclostomes appear to have evolved before dentine and bone, which are seen in numerous ancient agnathans, including conodonts. Whether cyclostomes are a clade is a topic of discussion with biologists. According to the "vertebrate theory," lampreys are more closely linked to gnathostomes than to hagfish. According to the "cyclostome theory," lampreys and hagfishes are more closely linked, making Cyclostomata monophyletic.

The vertebrate theory has been supported by the majority of anatomical investigations, whereas the cyclostome hypothesis has been supported by the majority of molecular phylogenies. However, there are exceptions in both cases.

The cartilage and muscles of the mouth apparatus are similarly similar in lampreys and hagfishes, indicating a sister-group connection. The vertebrate theory has been supported by at least one molecular phylogeny. Hagfish embryonic development was originally considered to be vastly different from that of lampreys and gnathostomes, but new data shows that it is more comparable than previously thought, potentially removing a roadblock to the cyclostome theory. There is currently no agreement on the proper topology.

Classification of Cyclostomes

There are over 50 different species of class Cyclostomata. They are divided into two groups (Petromyzontiformes and Myxiniformes). Subclasses, orders, and families are all terms used to describe them. They are various class Cyclostomata because they have circular jawless mouths. Convergent evolution is still most likely to blame for the similarities between these two groupings. They do, however, exhibit significant and fundamental physical differences, which can be linked to their long evolutionary isolation as well as their distinct behaviours and environments.

1. Petromyzontiformes (Gr., petros = stone; myzon = suck)

• Lampreys, lamper eels, lamperns, sand pride, and other names for members of this order exist.

• Ventral, suctorial mouth with rasping tongue encrusted with horny teeth.

• Dorsal nasopharyngeal na The nasohypophyseal sac ends in a blind sac, which means it does not interact with the pharynx.

• Below the oesophagus, there are seven pairs of gill-pouches and gill-slits that open into a distinct respiratory pharynx.

• The dorsal fin is quite well developed.

• The branchial basket is finished.

• The roots of the dorsal and ventral spinal nerves are kept distinct.

• Two semicircular canals in the ear.

• The eggs are many and tiny. With a lengthy larval stage and metamorphosis, development is indirect.

• There are both marine and freshwater varieties.

Cyclostomata examples of petromyzontiformes

Lampreys. Over 30 species. Petromyzon, Lampetra, Ichthyomyzon.

2. Myxiniformes (Gr., myxa = slime; oidea = type of)

• Hagfishes are representatives of the order.

• They only exist in the sea.

• 8 tiny tentacles surround the mouth terminal. Teeth are scarce. There is no buccal funnel.

• The terminal of the nasopharynx. In the pharynx, the nasohypophyseal sac opens posteriorly.

• 6 to 14 pairs of gill pouches and gill slits

• The dorsal fin is weak or non-existent.

• The branchial basket is underdeveloped.

• The dorsal and ventral roots of the spinal nerves have merged.

• Only one semicircular canal in the ear.

• Eggs are scarce and big. Dark development.

• All hagfishes are marine creatures.

Cyclostomata examples of myxiniformes

Hagfishes. About 15 species Myxine, Eptatretus (= Bdellostoma), Paramyxine.

Representative Types of Cyclostomata

1. Myxine

Hagfish is the common name for members of the Myxiniformes order. They only exist in the sea. Myxine is discovered buried at the bottom of the sea. Myxine is found in the seas of Northern Europe, the North Atlantic, America, Chili, Africa, and Japan, among other places, along the Atlantic and Pacific Ocean shores. The body resembles that of an eel, measuring around 2 feet (50-60 cm) in length and divided into three sections: head, trunk, and tail. 

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The body's surface is soft and smooth, with no scales. The mouth is at the end of the body and is encircled by soft lips. Jaws and a buccal funnel are missing. The branchial basket has been decreased as well. Four pairs of small tentacles flank the mouth and are supported by skeletal rods. The nostril is solitary, situated near to the mouth, and closes at the end. On the top of the skull, a single pineal eye may be seen. Due to their habit of living on the ground, paired eyes have become vestigial or degenerated. Six pairs of gills that do not open to the outside independently, but rather through a single pair of external gill holes. A single median ventral fin extends around the tail region from the middle of the ventral surface. Large mucus glands can be found on both sides of the body, opening through mucous pores and secreting mucus. Hermaphroditic having a single ovotestis, with the ovary in the front and the testis in the back. Because they are occasionally discovered within the bodies of their prey, which include various species of fish, these creatures are parasitic or quasi-parasitic. Feeders who feed at night. They spend the day buried in the muck of the seafloor at depths of over 2,000 feet. Hagfish do not migrate to spawn in freshwater.

2. Eptatretus (= Bdellostoma)

Hagfish is another name for Bdellostoma. It was discovered submerged in the sea's bottom muck. It may be found off the shores of North and South America, as well as South Africa and New Zealand, in the Pacific Ocean. The delicate, smooth integument of the long eel-like body is devoid of scales. It measures around one metre in length. The terminal mouth is encircled by soft lips. Jaws and a buccal funnel are missing. On the lateral edges of the mouth, there are four pairs of small tentacles supported by skeletal rods. 

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The single nostril is near to the mouth and closes at the end. On the top of the skull, there is a single pineal eye. Due to the bottom-dwelling lifestyle, paired eyes are vestigial or degenerated. There are 6-14 gill holes, each with circular pores that open to the outside. The pharyngocutaneous duct opens beneath the left side's final gill slit. The pharynx is where it opens. The middle fin is only seen in the caudal portion of the fish. On both sides of the body, large slime or mucous glands open through mucous pores. Hermaphroditic. The anterior portion of the ovary and the posterior testis make into a single ovotestis. It is parasitic or semi-parasitic. Feeders who feed at night. They spend the day buried in the muck of the seabed.

Affinities of Cyclostomes

Cyclostomes are chordate, as may be seen. They are the ancestors of all vertebrates. Their ammocoete larvae share several characteristics with Branchiostoma, indicating a primordial connection. Adult cyclostomes, on the other hand, have both specialised and degenerate features. 

1. Primitive Characters of Cyclostomes

• Jaws, exoskeleton, and paired fins are all missing.

• The notochord is continuous (but with an added sheath).

• Myotomes (segmental musculature) but little modification from head to tail.

• Scales, real teeth, true fin rays, girdles, ribs, stomach, spleen, and gonoducts are all missing.

• The middle dorsal fin is continuous.

• The caudal fin of a diphycercal fish.

• Instead of having two median nostrils, there is only one.

• A median pineal eye is present.

• There is a velar pump-like lamprey present.

• Sac-like endostyle.

• Although the nasal sacs are paired, there is only one nasal aperture.

• The brain is similar to that of a lamprey.

• In the ear, there are two semicircular canals.

• Up to 15 degrees divide the dorsal and ventral nerve roots.

2. Specialised Characters

• Lampreys have a sucking mouth buccal funnel and horny teeth for attachment.

• The rasping organ is a powerful tongue equipped with sharp horny teeth.

• To feed on prey blood, anticoagulants are secreted in saliva.

• Gill-pouches that look like sacs. Located behind the head. It's most likely a burrowing adaptation.

• The lower sac-like respiratory pharynx is completely separated from the top digesting pharynx.

• Water enters and exits gill pouches through external gill holes rather than the mouth.

• The presence of enormous mucus-secreting mucous glands in hagfishes.

• In lampreys, the nose on the head is in the dorsal position.

3. Degenerate Characters

• Hags have a more prominent simple elongated eel-like body.

• The body of an ostracoderm is wide and fish-like.

• In hags, rudimentary paired eyes are hidden by thick skin.

• Exoskeleton or bone armour isn’t present.

• There are no paired fins or girdles.

• It is cartilaginous due to the lack of ossification of the endoskeleton.

• Adult lampreys have a smaller liver and no gallbladder or bile duct.

Conclusion

The Cyclostomata (Greek: cyklos = round + stoma = mouth) originated in the Devonian as a modified and degenerate branch of the basic vertebrate stalk. They have a long, thin body with no scales or fins, a circular jawless mouth with horny teeth, and a cartilaginous skull. Myxine is found in the seas of Northern Europe, the North Atlantic, America, Chili, Africa, and Japan. Eptatretus is another name for Bdellostoma. They spend the day buried in the muck of the seafloor at depths of over 2,000 feet. The bottom-dwelling fish is parasitic or semi-parasitic. There are 6-14 gill holes, each with circular pores that open to the outside. Hermaphroditic fish spend the day buried in the muck of the seabed.