Cyclostomata: Definition, Meaning, Characteristics, Classifications, Examples

What are Cyclostomata?

The class Cyclostomata represents the jawless fish and consists of two basic divisions: hagfishes (Myxini), and lampreys (Petromyzontida). Among the distinct features of these animals are the absence of jaws and paired fins, which allow their study to be of central importance in understanding vertebrate evolution. Cyclostomes are among the most primitive living vertebrates, giving insight into vertebrate development at an early stage.

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Taxonomy And Classification Of Cyclostomata

• Kingdom: Animalia

• Phylum: Chordata

• Subphylum: Vertebrata

• Class: Cyclostomata

• Order: Myxiniformes (Hagfishes)

• Order: Petromyzontiformes (Lampreys)

Myxini (Hagfishes)

• Characterised by slime on their bodies and their scavenging lifestyle.

• Mostly marine species on dead or dying fish.

Petromyzontida (Lampreys)

• They are known to be parasites of some species, attaching themselves to other fish to suck their blood.

• It occurs in both marine and freshwater environments.

General Characteristics Of Cyclostomata

Body Structure: The cyclostomes are characterised by their elongated bodies, eel-like in shape, with no paired fins, and no jaws. The mouth is sucker-like and circular.

Skeleton: They have cartilaginous skeletons; that is, they are made of cartilage rather than bone. In addition to this, they do not have true vertebrae, only a notochord. The notochord is a long, flexible, rod-like structure that tends to help in supporting the animal.

Sensory Organs: The cyclostomes have rather poorly developed eyes and a lateral line system that detects vibrations and movements in the water. This enables one to steer through the medium of water and to search for food.

Anatomy And Physiology Of Cyclostomata

• Mouth: Circular, sucker-like structure with keratinised teeth (particularly in lampreys).

• Gill Slits: Multiple external openings along the sides of the body used for respiration.

• Nostril: Single median nostril located on the top of the head, used for olfaction.

• Eyes: Simple, lacking the complexity of higher vertebrates.

• Fins (if present): Some species may have rudimentary dorsal or caudal fins for stabilisation during swimming.

• Segmented Body: Cyclostomes have elongated, cylindrical bodies that are segmented.

• Setae are absent in cyclostomes, differentiating them from other segmented animals like annelids.

• Clitellum: Not present in cyclostomes, unlike in annelids such as earthworms.

Digestive System

• Their mouth is circular and suitably threshed for their mode of feeding.

• In lampreys, the mouth is furnished with rasplike teeth, which attach them to their hosts and by which they rasp out their food.

• Pharynx: The buccal cavity opens into a pharynx provided with numerous gill slits.

• Esophagus and Intestine: Food is passed from the pharynx into the oesophagus and then to the simple, straight intestine. A differentiated stomach is absent. The absorptions are done by the intestine and end in the cloaca.

• Cloaca: A common chamber into which the digestive, excretory, and reproductive systems open.

Respiratory System

• Gill Pouches: Several gill pouches occur, usually 5-16 pairs, in the body of the cyclostomes. Each opens outside as a separate gill slit. The pouch is lined by gill filaments which make up the respiratory surface or area of exchange of gases.

• Branchial Basket: This is the cartilaginous support that decorates gills in lampreys. It aids in the maintenance of the shape and function of the gill pouches.

Circulatory System

• Heart: The cyclostomes have a simple two-chambered heart with one atrium and one ventricle. It pumps deoxygenated blood into the gills for oxygenation.

• Blood Vessels: The major vessels, such as the dorsal and ventral aorta, branch into other arteries and veins and diffuse in the body to circulate blood.

• In the cyclostomes, both the hepatic and renal portal systems are highly developed, and their importance to blood filtration is absolutely at the forefront through the liver and kidneys.

Nervous System

• The brain of the cyclostomes is relatively simple and differentiated into a few regions: the fore-, mid-, and hindbrain, being enclosed in a cartilaginous cranium.

• Different sensory and motor functions are connected to the brain of the Cyclostomes by a series of cranial nerves.

• Extending from the brain, it obtains down through the length of the body within the notochord, where it coordinates sensory and motor information between the brain and body.

Sensory Organs

• Eyes: Simple eyes, only for perceiving light and movement, not in detail.

• Nostril: Single median nostril leading into the olfactory sac; for detecting chemicals in water.

• Lateral Line System: A series of mechanoreceptors along the body that allows fish to detect movements and vibrations in the water, facilitating navigation and prey detection.

Diagram Of The Structure Of Cyclostomata

The diagram below shows the different body parts of a Cyclostomata.

Life Cycle Of Cyclostomata

Cyclostomes are sexual reproducers. For example, lampreys demonstrate a complex life cycle characterised by metamorphosis from larva to adult.

The life cycle of a lamprey:

Eggs hatch as larvae, known as ammocoetes, which attach to organic matter from freshwater sediments.

Eventually, the larvae metamorphose into adult lampreys that may either migrate to the sea or remain in freshwater—interior anadromous species.

Habitat And Ecology

These cyclostomes inhabit a variety of habitats. Hagfishes are mostly deep-sea marine, while lampreys are both in marine and freshwater systems. Lampreys also make very long migrations from one habitat to the other.

Ecological Roles:

• Some of the species, like the hagfish, act as scavengers, devouring dead and decaying remains.

• The impact that lampreys, particularly their parasitic forms, have on fish populations involves sucking their blood and feeding on tissues.

• Lampreys can significantly affect fish populations, both through predation and parasitism.

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Evolutionary Significance Of Cyclostomata

Cyclostomes provide very important clues to the early vertebrate evolution. Fossils of ancient jawless fish show there was a stage of transformation from invertebrates to vertebrates.

The existence of cyclostomes marks an important step in the vertebrate lineage between more primitive chordates and jawed vertebrates. Comparative anatomy of the cyclostomes and higher vertebrates reveals the evolutionary steps taken toward the development of more complex vertebrates.

Human Interaction And Impact

It has various applications in human activity: hagfish slime is also researched for its potential uses in materials science; lampreys are used for food when people gather them now and then and in scientific research due to their unusual physiology.

Many of the species of cyclostomes are threatened by habitat destruction, pollution, and overfishing. It follows that protecting habitats and controlling pollution would secure their survival.