Agnatha – Characteristics, Classification, Morphology, Reproduction

Agnatha is the superclass of vertebrates in which true jaws are absent. It is the group that represent the most primitive and earliest craniates, and in this group the mouth parts are simple without any jaw elements. It is the condition where the animals show a very old vertebrate body plan which is already present from the early Cambrian period.

It includes both extinct armored forms called Ostracodermi and the living jawless forms that is the Cyclostomata (hagfish and lampreys). The body of these animals is characterized by the absence of paired fins, and the notochord is persistent throughout life which act as the main axial support. The skeleton is cartilaginous and the scales are absent in modern forms while the extinct groups had heavy bony plates.

In lampreys, a rudimentary vertebral element called arcualia is present around the notochord, but in hagfish these vertebral structures is not present. It is the group where the respiratory system contains 7 or more pairs of gill pouches and the circulatory system has a two-chambered heart. These animals are ectothermic and the alimentary canal has no distinct stomach.

Among the living members, hagfish are marine scavengers which produce slime for protection, while lampreys have a larval stage (ammocoete) that filter-feeds in freshwater and the adult can be parasitic with a sucking mouth. It is the process where molecular studies show that hagfish and lampreys form a single natural group referred to as Cyclostomata, and many similarities with jawed vertebrates are thought to be either convergent or lost in hagfish during evolution.

Class Ostracodermi

Ostracodermi is the earliest known group of vertebrates which appeared in the fossil records of late Paleozoic rocks. It is the group that lived around 500 million years ago and these forms are considered as the ancestors of all vertebrates. The animals lived in both marine and freshwater habitats and their body was fish-like with a dorso-ventrally flattened shape. It is the condition where paired fins are absent and only median fins is present for movement.

These animals are also identified by the presence of heavy bony dermal plates which covered the anterior part of the body giving protection. The sensory system was complex with the presence of large lateral eyes, a pineal eye and a single nostril, although much of the internal anatomy is not clearly known. It is the group where jaws are absent and the mouth is small and present on the ventral side which helped in filter-feeding as they were slow bottom dwellers. The internal ear had two semicircular canals which helped in maintaining balance. A lateral line system was also present which probably helped in detecting water currents and surrounding movements.

Some of the main divisions of Ostracodermi are–

Subclass I: Monorhina

Order Euphanerida (Example: Jamoytius kerwoodi)

Order Anaspida (Example: Lasanius, Birkenia)

Order Osteostraci (Example: Cephalaspis)

Subclass II: Diplorhina

Order Heterostraci (Example: Drepanaspis)

Order Coelolepida (Example: Coelolepis)

Class Cyclostomata 

Cyclostomata is the class of jawless vertebrates under Agnatha. It is the group where the mouth is round and suctorial, and the name itself comes from the Greek word cyclo meaning round and stomata meaning mouth. The body is elongated and eel-like, and this form helps the animals to swim with undulating movements and also attach to different substrates. It is the condition where paired fins are absent and only the median fin is present which is supported by cartilaginous fin rays. The tail is diphycercal because the vertebral axis runs straight to the tail making a symmetrical rounded structure.

The skin is soft without any scales and it secretes a thick mucus layer which helps in protection. The trunk and tail musculature is arranged into myotomes and the segments are divided by myocommata. The skeleton is cartilaginous with a persistent notochord which act as the main supporting axis. There is a rudimentary structure called arcualia that partly protects the nerve cord. In this group jaws are absent and the mouth is used for suction feeding.

The digestive system is simple, and no stomach is present. It is the process where the intestine contains a spiral fold called typhlosole that increases the surface area for absorption. The respiratory system contains 5 to 16 pairs of gills located inside sac-like pouches on the lateral side of the pharynx. This type of gill arrangement is termed Marsipobranchii. They also show 1 to 16 pairs of gill slits depending on species. The circulatory system has a two-chambered heart with an atrium and a ventricle. Cyclostomes are poikilothermic and the renal portal system is absent.

The excretory system has two mesonephric kidneys and the urinogenital papilla is the opening for both excretion and reproduction. The nervous system contains 8 to 10 pairs of cranial nerves and the dorsal nerve cord is expanded anteriorly into a simple brain. Among the important sensory structures, a single median olfactory sac and a median nostril is present. The internal ear contains one or two semicircular canals which help in balancing the body.

Reproductively, cyclostomes have a single gonad and the gametes are released into the body cavity through the urinogenital papilla. Fertilization is external. It is the condition where development includes a larval stage which later metamorphoses into the adult form.

General Characteristics of Class Agnatha

• It is the most primitive group of vertebrates where true jaws are absent.
• The paired fins like pectoral and pelvic fins is not present.
• The group includes extinct Ostracodermi and the living Cyclostomata.
• Body covering is different in both groups where Ostracoderms had heavy bony dermal plates and Cyclostomes have smooth scaleless skin.
• The main axial support is the persistent notochord which remains throughout life.
• The skeleton is fibrous and cartilaginous in nature.
• Lampreys contain rudimentary arcualia around the nerve cord but hagfish lack these structures.
• A single median nostril is present in Cyclostomes.
• Eyes may be well developed as in lampreys or vestigial as in hagfish.
• They are ectothermic animals.
• The heart is simple with two chambers (one atrium and one ventricle).
• Respiration takes place through many pairs of gill pouches or slits.
• The digestive canal is simple and no distinct stomach is present.
• Fertilization is external and development occurs without parental care.
• Feeding habits are different, lampreys are parasitic with sucking mouth while hagfish are scavengers.

Metabolism of Agnatha

• These are the most primitive living vertebrates without jaws and paired fins.
• It is the group that include hagfish and lampreys, which are called cyclostomes.
• The body is elongated and eel-like in structure.
• It is characterized by the absence of true bone and the endoskeleton is mostly cartilaginous.
• Notochord is persistent throughout life and it is the main axial support.
• The mouth is round without jaws, and it is referred to as a suctorial mouth in lampreys.
• Scales are absent and the skin is soft with mucous glands.
• These animals are ectothermic and their metabolism is slow in cold water.
• The heart is simple 2-chambered. It has one atrium and one ventricle.
• The circulation is of closed type.
• They lack a true stomach and the gut is long and uniform.
• In the intestine of cyclostomes a spiral fold (typhlosole) is present helping in digestion and absorption.
• Respiration is performed by gill pouches.
• Hemoglobin is present but its origin is considered independent from jawed vertebrates.
• Hagfish are deep-sea scavengers and shows very low energetic demand.
• Oxygen uptake in hagfish occurs cutaneously in resting condition.
• Hagfish are osmoconformers and the body fluid has almost same salinity with seawater.
• Lampreys are efficient osmoregulators. It can maintain blood osmolality about one-third of seawater.
• In freshwater lamprey gills take ions actively and kidneys excrete water.
• In seawater lampreys drink seawater and excess ions are excreted by gills.
• Adult lampreys are parasitic and attach to fish using toothed sucking mouth.
• Larval lampreys (ammocoetes) are filter feeders living in freshwater sediments.
• Some lamprey species have non-feeding adults that depend on the stored food from larval stage.

Morphology of Agnatha

• It is the group of vertebrates without jaws.
• Body generally elongated with soft skin.
• Paired fins is absent, only median fins are present.
• Notochord is persistent throughout life and it is the major supporting structure.
• Endoskeleton is cartilaginous in nature.
• Stomach is absent and the alimentary canal is straight.
• They respire by having 6–14 pairs of gill pouches.
• Scales is absent in modern forms.

Morphology of Extinct Agnatha (Ostracodermi)

• Body surface was covered by heavy dermal armour made up of bony plates.
• Head region had a large bony shield which protected the gill region.
• Mouth was small and jawless, situated ventrally.
• Feeding was done by sucking action with the help of muscular pharynx.
• Eyes was lateral and a median pineal eye was also present.
• Sensory fields formed part of lateral line system.
• Nostrils were either single median (Monorhina) or paired (Diplorhina).
• Internal ear had two semicircular canals.

Morphology of Extant Agnatha (Cyclostomata)

A. Lampreys (Petromyzontiformes)

• Body elongated and eel like.
• Notochord is persistent and surrounded by cartilaginous arcualia.
• Skeleton is fibrous and cartilaginous.
• Mouth is a round sucking disc called buccal funnel.
• Teeth are horny and arranged in radiating rows.
• A piston like tongue is present for rasping tissues.
• One median nostril is present.
• Eyes are well developed and a pineal eye is present.
• Seven gill pores is present on each side.
• Intestine has a spiral fold called typhlosole.
• Heart is simple with two chambers.

B. Hagfish (Myxiniformes)

• Body elongated, scaleless and soft.
• Notochord is present throughout life and vertebral elements is absent.
• Cranium is present but no vertebral column.
• Mouth is ventral with 4 or 8 barbels.
• Keratinous tooth plates are present for biting.
• Eyes are vestigial and covered by skin.
• Barbels and chemoreception help in sensing food.
• They produce copious slime from slime glands.
• Body is flexible and can form knot for scraping slime.
• Gill openings vary according to species (5–15 pouches).

Reproduction of Agnatha

Reproduction of Agnatha (Simple List Format)

• It is the process by which jawless vertebrates (Agnatha) form new individuals.
• Fertilization is external in general.
• Parental care is absent.
• Cyclostomes possess a single large gonad.
• Sexes are separate.
• Development is external.

Lamprey Reproduction

• Lampreys show an anadromous life cycle.
• They are semelparous and die after spawning.
• Adults migrate from sea to freshwater for spawning.
• A nest (redd) is made in sand or gravel.
• The male attaches to the female by the buccal funnel.
• Female releases thousands of eggs which are fertilized externally.
• Eggs are covered with sand by tail movements.
• Adults die after a few days.
• Mature males have heat-producing tissue near the dorsal fin.
• Pheromones help in attraction of mates.
• Alimentary canal becomes non-functional during migration.

Ammocoete Larvae

• Development is indirect.
• Eggs hatch into ammocoetes.
• Ammocoetes burrow into soft sediment.
• They are filter feeders.
• Larval life lasts 3–15 years.
• Metamorphosis changes the larva into the adult with eyes and sucking mouth.

Hagfish Reproduction

• Reproduction is not fully known.
• Fecundity is very low.
• Females carry 20–40 large eggs.
• Eggs have gelatinous caps with hooks.
• Eggs may attach in strings.
• Development is direct.
• Hatchlings measure 45–58 mm.
• Some species are hermaphroditic.
• Juveniles may show protandry.
• Fertilization may be internal due to low sperm amount.

Evolution of Agnatha

Agnatha is the group of jawless vertebrates. It is the earliest and most primitive assemblage in Vertebrata. It is characterised by the absence of jaws and paired appendages. The modern representatives are hagfish and lampreys which is referred to as Cyclostomes. The extinct members were diverse and heavily armored.

Early Appearance

The origin of Agnatha is traced to Cambrian time. It is believed that the earliest vertebrate-like forms appeared in Lower Cambrian deposits. Some of the initial fossils include Haikouichthys and Myllokunmingia which shows primitive vertebrate characters.

Conodont animals were also present in early Cambrian. These are considered as soft-bodied jawless agnathans. Conodont elements (phosphatic structures) is the only mineralized part. These forms survived till Triassic and it is accepted that they were closer to vertebrates.

Ostracoderms

Ostracoderms were armored jawless fishes. It appeared during Middle Ordovician (about 470 million years ago). These forms reached the major development during Silurian period.

These forms possessed heavy dermal armor. The armor plates were embedded in the dermis. It served as protection and also helped in ionic balance. Some of the groups like Osteostraci developed paired fins (flappers) which is an important evolutionary feature.

Ostracoderms used gills only for respiration. This is considered a major transition as earlier chordates used pharynx mainly for filter feeding.

The group declined during Devonian time and completely got extinct at the end of Devonian.

Transition to Gnathostomes

It is the process by which jawed vertebrates evolved from the jawless ones. This is believed to occur in Early Silurian period. The first gill arches developed folding articulations which formed jaws. Many of the ostracoderm groups like thelodonts and osteostracans were more closely related to gnathostomes than present cyclostomes.

Evolution of Cyclostomes

Cyclostomes include hagfish (Myxini) and lampreys (Petromyzontida). These are the only surviving agnathans. These forms lack heavy bony armor and possess a suctorial mouth.

It is considered that modern cyclostomes evolved from cephalaspid-like ancestors by reduction of bony armor, paired fins, and by developing specialized feeding structures.

Fossil Records

Some important fossil evidences are–

– Priscomyzon from Late Devonian about 360 million years ago shows lamprey-like form.
– Hagfish-like fossil Myxinikela is reported from Pennsylvanian period (280 million years).
– Yanliaomyzon from Middle Jurassic indicates predatory habit around 163 million years ago.

Molecular data suggests that cyclostomes and gnathostomes separated in Early Cambrian during the major radiation event.

Cyclostome Phylogeny

There are two hypotheses regarding their relationships.

Morphological Hypothesis

It is believed earlier that lampreys are closer to gnathostomes than hagfish. This made Agnatha a paraphyletic group. Lampreys possess two semicircular canals, better osmoregulation, functional eyes and rudimentary vertebral elements (arcualia). Hagfish lacks these structures and was considered as a degenerate form of vertebrates.

Molecular Hypothesis

Molecular studies (DNA, rRNA, microRNA) strongly support that hagfish and lampreys form a monophyletic group. This group is called Cyclostomata. This is considered as a sister group to gnathostomes. The similarities between lampreys and gnathostomes are now interpreted as either convergent or lost in hagfish during specialization.

Special Evolutionary Features

Some important evolutionary features of cyclostomes are described below–

1. Skeletal Divergence
   Lampreys retained notochord and arcualia, which is an early form of vertebra. Hagfish lacks arcualia and depends completely on notochord.
2. Loss of Hard Tissues
   Cyclostomes diverged before the evolution of dentine and true bone. Many fossil agnathans possessed these tissues but modern forms do not have them.
3. Evolution of Hemoglobin
   Hemoglobin formation occurred independently in jawless and jawed lineages by gene co-option.
4. Ammocoete Reinterpretation
   Earlier it was considered that ammocoete larva represents primitive vertebrate condition. But fossils of stem lampreys show that early larvae resembled miniature adults. Thus the filter-feeding larva is a later evolutionary stage.
5. Degeneracy and Specialization
   Eyes in hagfish are vestigial because early hagfish ancestor (Myxinikela) possessed well-developed eyes. Hagfish has also lost large number of genes (about 1386 families) which shows extreme specialization.

Classification of Agnatha

Agnatha is the jawless group under early vertebrates. It is divided into different subgroups based on their structure, fossil records and evolutionary features. These are the oldest representatives of vertebrate lineage. The main groups are described below.

1. Cyclostomes

These are the only living jawless vertebrates. It includes hagfish and lampreys. It is the group where circular mouth is present.

Example – Myxini (Hagfish)

Hagfish are eel-like marine forms. It possesses a skull but the vertebral column is absent. It produces large amount of slime and lives in deep marine water. It is considered a living fossil. The classification of hagfish has been debated because some studies suggested that hagfish is a degenerate vertebrate, while others believed they represent a primitive condition before the evolution of vertebra.

Earlier, hagfish and lamprey were placed together under Cyclostomata. Recent DNA studies support that this grouping is correct and both represent a monophyletic group.

2. Hyperoartia

It is the group which includes modern lampreys and fossil lamprey-like forms.

Example – Lamprey

Lampreys have a long fossil history. Some early fossil genera like Endeiolepis and Euphanerops lived during Late Devonian period. They possess a suctorial mouth, notochord and cartilaginous skeleton. Their exact position among jawless groups is still debated. At first they were grouped with hagfish as cyclostomes. But some studies indicates that lampreys may be closer to some ostracoderm forms. The correct placement is still under study.

3. Myllokunmingiida

This is one of the earliest known agnathan groups.

Example – Haikouichthys

These forms are from Cambrian Maotianshan Shales of China. It is the earliest known craniates. These organisms possessed a skull made of cartilage or bone. Important genera are Haikouichthys, Myllokunmingia and Zhongjianichthys. These forms represent the primitive stage of vertebrate evolution.

4. Conodonta

Conodonts are extinct jawless forms. They existed from Cambrian to Early Jurassic.

Example – Conodont Animals

These are eel-like organisms. They were first known only from microscopic dental elements known as conodont elements. Later body fossils were found. These forms acted as excellent index fossils. They had peaks of diversity during middle Ordovician and mid to late Devonian. They declined in Carboniferous and again increased during early Triassic before final extinction.

The lifestyle varied. Some were filter feeders while others were active predators.

5. Ostracoderms

Ostracoderms are extinct armored jawless fishes.

Examples – Pteraspidomorphi, Thelodonti, Anaspida, Cephalaspidomorphi

These forms possessed heavy dermal armor and lived in both freshwater and marine environments. Many forms had hypocercal tail which helped in movement. The armor was thick and covered the head and body. Some groups like Heterostraci, Astraspida and Arandaspida belong under Pteraspidomorphi.

Ostracoderms appeared in Ordovician and became extinct at the end of Devonian period. They were important forms which occupied different ecological habitats such as reef regions and shallow marine areas.

FAQ

What are Agnatha?

Agnatha are the jawless vertebrates. It is the group that includes both extinct jawless fishes and the living cyclostomes.

What are the characteristics of Agnatha?

These are vertebrates without jaws. The mouth is circular. Paired fins are absent. The notochord is persistent. The skeleton is cartilaginous. Scales may be absent. They show primitive vertebrate characters.

What are examples of Agnatha?

Examples are lampreys and hagfish.

Do Agnatha have jaws?

No. The jaws are absent.

What are the two living groups of jawless fish?

The two living groups are lampreys (Petromyzontida) and hagfish (Myxini).

What is the difference between Agnatha and Gnathostomata?

Agnatha lack jaws and paired fins. Gnathostomata possess jaws and paired appendages. Agnatha show primitive features. Gnathostomata show advanced vertebrate organization.

Do Agnatha have paired fins?

No. Paired fins do not occur.

What is the skeleton of Agnatha made of?

The skeleton is cartilaginous and the notochord is the main supporting structure.

How are Agnatha classified?

Agnatha are classified into extinct armored forms (Ostracoderms) and the living Cyclostomata which include lampreys and hagfish.

When did Agnatha first appear?

They first appeared in the Cambrian period.

Do Agnatha have a stomach?

No. A true stomach is absent in most Agnatha.

What do lampreys eat?

Parasitic lampreys feed on the body fluids of host fishes using their sucking mouth and teeth.

What do hagfish eat?

Hagfish feed on dead or dying fishes and other soft-bodied animals. They enter the body and consume internal tissues.

Do Agnatha have scales?

Most living Agnatha do not have scales. The extinct forms had bony plates or armor.

What is a notochord in Agnatha?

The notochord is the flexible supporting rod present throughout life. It forms the main axial support because vertebral elements are absent or very reduced.