The group is mainly divided into two extant orders—Petromyzontiformes (lampreys) and Myxiniformes (hagfishes)—based on differences in feeding behavior, reproductive methods, and anatomical. The cyclostomes are subdivided into two major orders.
Cyclostomata
Edited By Irshad Anwar | Updated on Jul 02, 2025 06:03 PM IST
Cyclostomata is a group of jawless vertebrates known for their long slender eel-like bodies and round sucker like mouths. They don't have jaws or paired fins and use their mouth, either attaching to other fish or for filter feeding. Common examples include Lampreys and hagfish. These animals belong to the superclass Agnatha, which includes all jawless fish. Their skeleton is made of cartilage, not bone, making them more primitive than other vertebrates. Cyclostomata is an important topic in class 11 Biology under the chapter of the animal Kingdom and often appears in exams due to unique features in classifications.
This Story also Contains
- What are Cyclostomata?
- Taxonomy and Classification of Cyclostomata
- Cyclostomata Origin
- Subdivisions of Cyclostomata
- Cyclostomata Characteristics
- Anatomy and Physiology of Cyclostomata
- Habitat and Ecology
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What are Cyclostomata?
The class Cyclostomata represents the jawless fish and consists of two basic divisions. These are hagfishes (Myxini) and lampreys (Petromyzontida). These animals are the absence of jaws and paired fins, which allows their study to be of major importance in understanding vertebrate evolution. Cyclostomes are among the most primitive living vertebrates, which help us to understand vertebrate development at an early stage.
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Taxonomy and Classification of Cyclostomata
Cyclostomata is a superclass under the subphylum Vertebrata and includes two main living classes: Petromyzontida (lampreys) and Myxini (hagfishes). These are grouped together due to their shared jawless condition and similar morphology. The taxonomy and classification of cyclostomata are described below-
| Kingdom | Animalia |
| Phylum | Chordata |
| Subphylum | Vertebrata |
| Class | Cyclostomata |
| Order | Myxiniformes (Hagfishes) |
| Order | Petromyzontiformes (Lampreys) |
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Cyclostomata Origin
Cyclostomata are jawless vertebrates that represent one of the most primitive groups of living chordates. They are believed to have evolved from early vertebrate ancestors during the Paleozoic era. Below are a few points on the origin of Cyclostomata:
- They are ancient vertebrates that appeared 500 million years ago during the early Palaeozoic era.
- They evolved from early vertebrate ancestors and represent one of the most primitive forms of living vertebrates.
- Cyclostomes are part of the Agnatha superclass, which means jawless fish, and they existed even before jawed fish evolved.
- Their skeleton is made of cartilage, and a simple body structure suggests that they branched off early from the early vertebrate line.
- Fossil records of ancient jaws fish help scientists trace the evolution of the vertebral column, nervous system, backbone, and brain.
Subdivisions of Cyclostomata
Myxini (Hagfishes)
Hagfishes are soft-bodied, jawless animals that produce large amounts of slimy mucus when disturbed, which helps them escape predators.
They are scavengers, meaning they feed on the bodies of the dead or dying animals by burrowing into them and eating from the inside.
Hagfishes live mainly in the marine environment and are rarely seen because they stay deep in the ocean.
Examples- Paramyxine and Myxine
Petromyzontida (Lampreys)
Petromyzon are parasitic, meaning attach themselves to other living fish using their sucker like mountains and feed on their blood.
They have a tooth-like mouth to latch onto their host.
Lampreys are found in both freshwater and marine habitats, and some species even migrate between the two.
Examples- Lampetra and Petromyzon
Cyclostomata Characteristics
Cyclostomes are jawless, eel-like vertebrates with a soft, elongated body and a circular, sucker-like mouth. They lack paired fins, jaws, and scales. Some general characteristics of Cyclostomata are discussed below:
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 support 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.
Respiration: The cyclostomes breathe through gills, which are present in pairs of gill slits located on the sides of the head. These gills are used for aquatic respiration and are covered by a soft skin-like structure instead of hard gill covers.
Circulatory system: They have a closed circulatory system with a two-chambered heart. Blood flows in one direction, unlike higher vertebrates. The red blood cells are usually nucleated. The circulation is simple but efficient for their body's needs.
Nervous System: Cyclostomes have a simple but well-developed nervous system with a brain and a spinal cord. Their sense organs are adapted to their aquatic and parasitic life cycle, including a single median nostril and eyes that may be poorly developed in some species.
Reproduction: Cyclostomes are unisexual, meaning, male and female reproductive parts are separate. Fertilisation is external, and they do not have larval care. The reproductive cycle involves migration to freshwater for spawning, after which the adult usually dies.
Anatomy and Physiology of Cyclostomata
The cyclostomes have a mouth, gill slits and nostrils that help in respiration and eyes to help them see. The general anatomy and physiology of Cyclostomata are discussed below:
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 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.
Habitat and Ecology
These cyclostomes inhabit a variety of habitats. Hagfishes are mostly deep-sea marine, while lampreys are found in both 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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Frequently Asked Questions (FAQs)
1. What is a Cyclostomata also known as?
Cyclostomatida, or cyclostomata (also known as cyclostomes), are an ancient order of stenolaemate bryozoans which first appeared in the Lower Ordovician. It consists of 7+ suborders, 59+ families, 373+ genera, and 666+ species.
2. What are the main differences between hagfish and lampreys?
Hagfish are scavenging animals that are only found in marine habitats, whereas lampreys are parasitic and have anadromous lifecycle stages, that is, they migrate between freshwater and marine environments.
3. Why are Cyclostomata important in evolutionary biology?
The Cyclostomata, therefore, represent an early phase in vertebrate evolution and thus give some insight into the transition from invertebrates to vertebrates.
4. How do lampreys reproduce?
Lampreys reproduce sexually, with external fertilisation occurring in freshwater environments, where they lay eggs that hatch into larvae.
5. What is the ecological significance of Cyclostomata?
Cyclostomata play a role in the food web as both predators and prey, and their parasitic behaviours can impact other fish species.
6. How do cyclostomes differ from other fish in terms of their mouth structure?
Cyclostomes have a circular, sucker-like mouth (hence the name "cyclo-stome," meaning "round mouth") instead of jaws. This mouth is lined with horny teeth and is used for attaching to prey or hosts. Unlike other fish, cyclostomes cannot bite or chew their food; instead, they use their tongue-like structure to rasp flesh from their prey or host.
7. How do cyclostomes breathe, and how does this differ from other fish?
Cyclostomes have a unique respiratory system consisting of multiple gill pouches. In hagfish, these pouches open directly to the outside, while in lampreys, they open into a common branchial chamber. This differs from the gill structure of other fish, which typically have gill arches and opercula (gill covers). Cyclostomes' respiratory system is considered more primitive and less efficient than that of other fish.
8. What is the function of the notochord in cyclostomes, and how does it compare to the vertebral column in other vertebrates?
The notochord in cyclostomes serves as the primary axial support structure, running the length of the body. It provides flexibility and support for swimming. In hagfish, the notochord persists throughout life, while in lampreys, it is partially replaced by cartilaginous vertebral elements. In more advanced vertebrates, the notochord is largely replaced by the vertebral column during development, serving primarily as a signaling center for embryonic patterning.
9. What are some unique sensory adaptations found in cyclostomes?
Cyclostomes have developed several unique sensory adaptations. They possess a well-developed olfactory system, with a single nostril (in hagfish) or two nostrils (in lampreys) leading to olfactory sacs. Lampreys have light-sensitive eyes, while hagfish have simple eyespots. Both groups have a lateral line system for detecting water movements and vibrations. Some species of hagfish also have specialized slime glands that produce a defensive mucus when threatened.
10. Why is the study of cyclostomes important for understanding vertebrate evolution?
Cyclostomes are crucial for understanding vertebrate evolution because they represent the most basal living vertebrates. By studying their anatomy, physiology, and genetics, scientists can gain insights into the early stages of vertebrate evolution and the development of key vertebrate features. Cyclostomes serve as a living link between invertebrate chordates and more advanced vertebrates.
11. How do cyclostomes contribute to our understanding of the evolution of the vertebrate heart?
Cyclostomes possess a simple two-chambered heart, consisting of one atrium and one ventricle. This heart structure is more primitive than the three-chambered heart of amphibians or the four-chambered heart of mammals and birds. Studying the cyclostome heart provides insights into the early stages of vertebrate heart evolution. It helps scientists understand how the multi-chambered hearts of more advanced vertebrates may have evolved from a simpler ancestral form. Additionally, the study of cyclostome heart development and function can reveal conserved genetic pathways important in vertebrate heart formation.
12. How do cyclostomes move, and what structures enable their locomotion?
Cyclostomes move primarily through anguilliform (eel-like) swimming motions. They lack paired fins but have a caudal fin and, in lampreys, dorsal fins. Their movement is facilitated by their flexible notochord and well-developed longitudinal muscles. The circular mouth of lampreys can also be used for attachment during upstream migration. Hagfish are known for their ability to tie themselves in knots, which helps them generate force for movement and escape from predators.
13. How do cyclostomes detoxify and excrete waste products, and what makes their excretory system unique?
Cyclostomes have a unique excretory system that differs from other vertebrates. They possess primitive kidneys called archinephros or holonephros, which are less complex than the kidneys of jawed vertebrates. In hagfish, the kidneys are segmentally arranged along the body cavity. Lampreys have a more compact kidney structure. Both groups use ammonia as their primary nitrogenous waste product, which is directly excreted through their gills and skin. This system is considered more primitive than the urea-based excretion found in many other aquatic vertebrates.
14. What role does the endocrine system play in cyclostome physiology, and how does it compare to that of other vertebrates?
The endocrine system of cyclostomes is more primitive compared to that of jawed vertebrates but still plays crucial roles in their physiology. They possess several hormone-producing glands, including the pituitary, thyroid, and gonads. However, they lack some endocrine organs found in higher vertebrates, such as the pancreas and adrenal glands. Despite these differences, cyclostomes use many of the same hormones found in other vertebrates for processes like growth, metabolism, and reproduction, highlighting the early evolution of endocrine signaling in vertebrates.
15. What is the significance of the cyclostome nervous system in understanding brain evolution?
The cyclostome nervous system provides valuable insights into the early evolution of the vertebrate brain. Cyclostomes have a simple but well-defined brain with major divisions similar to those found in other vertebrates, including the forebrain, midbrain, and hindbrain. However, their brain lacks certain structures present in jawed vertebrates, such as a cerebellum. Studying the cyclostome nervous system helps scientists understand the basic blueprint of the vertebrate brain and how it evolved to become more complex in higher vertebrates.
16. What are cyclostomes and why are they considered primitive vertebrates?
Cyclostomes are jawless fish that represent the most primitive living vertebrates. They are considered primitive because they lack key features found in more advanced vertebrates, such as jaws, paired fins, and a mineralized skeleton. Cyclostomes have a simple body plan and retain many ancestral characteristics, making them important for understanding vertebrate evolution.
17. What is the importance of the cyclostome life cycle in understanding vertebrate development and evolution?
The cyclostome life cycle, particularly that of lampreys, is important for understanding vertebrate development and evolution because:
18. How do the feeding mechanisms of hagfish and lampreys differ, and what does this tell us about their evolutionary history?
Hagfish and lampreys have different feeding mechanisms that reflect their evolutionary history. Hagfish are primarily scavengers and use their tooth-like structures to rasp flesh from dead or dying animals. They can also absorb nutrients through their skin. Lampreys, on the other hand, are often parasitic as adults, using their sucker-like mouth and rasping tongue to attach to and feed on live fish. These differences suggest that hagfish and lampreys diverged early in cyclostome evolution, adapting to different ecological niches.
19. What is the importance of slime production in hagfish, and how does this adaptation function?
Slime production is a unique and important adaptation in hagfish. When threatened, hagfish can produce copious amounts of slime from specialized glands along their body. This slime serves multiple functions:
20. What is the significance of the cyclostome skeletal system in understanding vertebrate evolution?
The cyclostome skeletal system is significant in understanding vertebrate evolution because:
21. How do cyclostomes reproduce, and what makes their reproductive strategy unique?
Cyclostomes have a unique reproductive strategy. They are generally semelparous, meaning they reproduce only once in their lifetime. Hagfish and lampreys have separate sexes and undergo external fertilization. Lampreys have a complex life cycle involving a larval stage (ammocoete) that undergoes metamorphosis into an adult. This reproductive strategy, combined with their primitive features, sets cyclostomes apart from other vertebrates.
22. What role do cyclostomes play in aquatic ecosystems?
Cyclostomes play important roles in aquatic ecosystems. Lampreys, as both predators and prey, contribute to nutrient cycling and energy flow in food webs. Some lamprey species are anadromous, migrating between freshwater and marine environments, thus connecting these ecosystems. Hagfish are important scavengers in deep-sea environments, consuming dead and dying animals on the ocean floor. They also play a role in bioturbation, which affects sediment chemistry and benthic habitats.
23. How do cyclostomes osmoregulate, and why is this process important for their survival?
Cyclostomes have developed unique osmoregulatory mechanisms to maintain their internal salt balance. Marine hagfish are osmoconformers, meaning their body fluids are isotonic with seawater. They have specialized cells in their gills that can absorb or excrete ions as needed. Lampreys, which can live in both freshwater and marine environments, are osmoregulators. They have specialized chloride cells in their gills that help maintain ion balance. This ability to osmoregulate is crucial for their survival in different aquatic environments.
24. How do cyclostomes regulate their buoyancy, and why is this important?
Cyclostomes lack a swim bladder, which is used by many fish for buoyancy control. Instead, they rely on their low-density tissues and lipid content to maintain buoyancy. Hagfish, in particular, have a subcutaneous sinus system filled with seawater that helps in buoyancy regulation. This ability to control buoyancy is crucial for their survival in various water depths and for energy-efficient swimming.
25. What is the ecological impact of invasive lamprey species, and how does this relate to their biology?
Invasive lamprey species, particularly the sea lamprey in the Great Lakes, have had significant ecological impacts. Their parasitic feeding behavior can severely damage or kill host fish, affecting native fish populations and ecosystem balance. The success of invasive lampreys is related to their biology, including their ability to adapt to new environments, their efficient reproductive strategy, and their lack of natural predators in invaded ecosystems. Understanding lamprey biology is crucial for developing effective control measures.
26. What are the two main groups of cyclostomes, and how do they differ?
The two main groups of cyclostomes are hagfish (Myxini) and lampreys (Petromyzontiformes). Hagfish are considered more primitive and lack a true vertebral column, while lampreys have a cartilaginous vertebral column. Hagfish are exclusively marine, while lampreys can be found in both marine and freshwater environments. Additionally, lampreys undergo metamorphosis during their life cycle, while hagfish do not.
27. How do cyclostomes contribute to our understanding of the evolution of the adaptive immune system?
Cyclostomes contribute significantly to our understanding of adaptive immune system evolution:
28. How do cyclostomes navigate in their aquatic environments, and what sensory systems are involved?
Cyclostomes use a combination of sensory systems to navigate their aquatic environments:
29. What is the significance of cyclostome genome studies in understanding vertebrate evolution?
Genome studies of cyclostomes are crucial for understanding vertebrate evolution. As the most basal living vertebrates, their genomes provide insights into the genetic basis of early vertebrate traits. Comparative genomics between cyclostomes and other vertebrates helps identify conserved genes and regulatory elements important for vertebrate development and function. These studies also shed light on the genomic changes that occurred during the transition from invertebrate chordates to vertebrates.
30. What is the significance of the cyclostome immune system in comparative immunology?
The immune system of cyclostomes is of great interest in comparative immunology because it represents the most primitive adaptive immune system among vertebrates. Cyclostomes possess lymphocyte-like cells and can produce antibody-like molecules, but they lack the diverse antibody repertoire found in jawed vertebrates. Studying their immune system provides insights into the evolution of adaptive immunity and the development of more complex immune responses in higher vertebrates.
31. How do cyclostomes cope with extreme environments, particularly in the case of hagfish?
Hagfish are known for their ability to thrive in extreme environments, particularly in the deep sea. They can tolerate low oxygen levels and high pressures found in these habitats. Hagfish have a low metabolic rate and can absorb nutrients through their skin, allowing them to survive in nutrient-poor environments. Their ability to produce copious amounts of slime also serves as a defense mechanism against predators in these challenging environments.
32. How do cyclostomes maintain their body temperature, and what adaptations allow them to survive in various aquatic environments?
Cyclostomes are ectothermic, meaning their body temperature is regulated by the surrounding environment. They lack the ability to generate significant internal heat. To survive in various aquatic environments, cyclostomes have developed several adaptations. These include the ability to adjust their metabolic rate in response to temperature changes, the production of antifreeze proteins to prevent ice formation in cold waters, and behavioral adaptations such as seeking out optimal temperature zones within their habitat.
33. What are some unique features of cyclostome blood, and how do they relate to their physiology?
Cyclostome blood has several unique features:
34. How do cyclostomes cope with changes in water salinity, and what does this tell us about osmoregulation in early vertebrates?
Cyclostomes have developed different strategies to cope with changes in water salinity:
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