Transcript Agnatha

 Scientific classification
 Kingdom:Animalia
 Phylum:Chordata
 Subphylum:Vertebrata
 Class:Agnatha
INTRODUCTION
Agnatha - (Greek, "no jaws")
 is a class or superclass of jawless fish in the phylum
Chordata, subphylum Vertebrata. The group excludes all
vertebrates with jaws, known as gnathostomes.
 as a whole are paraphyletic.
 The oldest fossil agnathans appeared in the Cambrian,
and two groups still survive today .
-Modern agnathans are characterized by absence
of paired fins; the presence of a notochord both
in larvae and adults; and seven or more paired
gill pouches.
- There is a light sensitive pineal eye
(homologous to the pineal gland in mammals).
- All living and most extinct Agnatha do not
have an identifiable stomach or any appendages.
Fertilization and development are both external.
There is no parental care in the Agnatha class.
The Agnatha are exothermic, with a cartilaginous
skeleton, and the heart contains 2 chambers
Key feature of Agnatha
 Jaws are absent.
 Paired fins are generally absent.
 Early species had heavy bony scales and plates in their
skin, but these are not present in living species.
 In most cases the skeleton is cartilaginous. The
embryonic notochord persists in the adult.
 Seven or more paired gill pouches are present.
 The digestive system lacks a stomach.
OVERVIEW AND CLASSIFICATION

Vertebrates (subphylum Vertebrata) are generally
classified into two groups:
 1.Agnatha (jawless vertebrates) - includes the modern day
lampreys (Petromyzontiformes) and hagfish (Myxiniformes) as
well as several extinct orders.
2. Gnathostomata (jawed vertebrates)- includes fish with
hinged jaws and the tetrapods (amphibians, reptiles,
birds, and mammals.
DESCRIPTION
 All living and most extinct agnathans do not have an
identifiable stomach or any paired appendages, although
the hagfish and lampreys do have a tail and a caudal fin.
 Some extinct agnathans reveal thick body plates.
 The internal skeleton of the Agnatha is not bony but
rather cartilaginous
-As characteristic of the class, hagfish and
lampreys have a notochord that remains throughout
life.
-This notochord is the first primitive vertebral
column. In the extant agnathans, fertilization and
development are both external, and there is no
parental
care.
-Although lampreys and hagfish are superficially
similar, many of these similarities are probably
shared
primitive
characteristics
of
ancient
vertebrates. Thus, modern classifications tend to
place hagfish into a separate group, with the
lampreys (Hyperoartii) being more closely related
to
the
jawed
fishes.
2 main types of fish in the
agnatha class.
1. LAMPREY
 are of the order, Petromyzontiform. They are suckers and attach
themselves to fish in order to parasitize off them.
 scaleless, eel-like, cartilaginous fish that have a round, jawless
mouth which suctions onto the flesh of the fish they feed on.
 their fins are not paired, and they have one nostril and a third pineal
eye on top of their head.
 they have 7 gills on each side
LAMPREYS LIFE CYCLE
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Lampreys will lay their eggs in flowing water.
When the eggs hatch, the larvae drift down-stream to an area with a soft, but not
muddy, bottom.
They dig into the bottom and form a burrow with their mouth just above the
bottom.
A good portion of the lifespan of the animal is spent in this filter-feeding larval
stage known as an ammocoete. As with most larvae, they do not yet have sex
organs.
When the metamorphosis to adult occurs, the lamprey swims into the water.
The Sea Lamprey will search for a host to feed on blood before mating.
Some Lamprey's do not eat as an adult. They mate and die
three weeks
in one to
Example of lampreys
2. Hagfish
 Are of the order, Myxiniform. They are related to the slimefish..
 Rather worm-like in appearance, the hagfish lack paired fins, only
having a slight tail fin.
 They have no eyes and no scales.
 They also have rows of horny teeth on the tongue which rasp at the
food.
 Surrounding the mouth is a ring of tentacles.
 Hagfish produce a great deal of slime
Example of a hagfish
Metabolism

Agnathans are ectothermic or cold blooded, meaning they do not regulate their own body
temperature.

Agnathan metabolism is slow in cold water, and therefore do not have to eat as much.

They have no distinct stomach, but rather a long gut, more or less homogenous throughout its
length.

Lampreys are parasitic, feeding off of other fish and mammals. They rely on a row of sharp teeth
to shred their host. Fluids preventing clotting are injected into the host,causing the host to yield
more blood.

Hagfish are decomposers, eating mostly dead animals. They also use a sharp set of teeth to break
down the animal.

Agnathans feeding habits have limited their ability to advance evolutionarily. The fact that all
Agnathan's teeth are not able to move up and down limit their possible food types. They have
never been known to attack humans, without water they would die so when they are taken out of
the water with their host they will release immediately.
Body covering
 The only modern Agnathan body covering is
skin, with neither dermal or epidermal scales.
 The skin of hagfish has copious slime glands, the
slime constituting their defence mechanism.
 Many extinct agnathans sported heavy dermal
armour or small mineralized scales (see below).
Appendages
 Most agnathans, including all those living today
have no paired appendages, although they do
have a tail and a caudal fin.
 Some fossil agnathans, such as osteostracans,
did have paired fins, a trait inherited in their
jawed descendants
Skeleton
 The internal skeleton of the Agnatha is not bony but rather
cartilaginous (made up of dense connective tissue).
 The somewhat rudimentary skull never ossifies and remains a
chondrocranium throughout life.
 Also, Agnathans retain a notochord in adulthood, a characteristic
distinctive of the class. This notochord is a cartilagious rod that
forms the basis of the vertebral column in highter vertebrates
Reproduction

Fertilization is external, as is development. There is no parental care.
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Not much is known about the hagfish reproductive process. It is believed that hagfish
only have 30 eggs over a lifetime.
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Most species are hermaphrodites. There is very little of the larval stage that
characterizes the lamprey.
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Lampreys can only reproduce once. After external fertilization, the lamprey's cloacas
remain open, allowing a fungus to enter their intestines, killing them.
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Lampreys reproduce in freshwater river beds, and bury their eggs about two
centimeters underground.
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Lampreys work in pairs buildings the egg nests.
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Lampreys go through four years of larval development before becoming adults. They
also have a certain unusual form of reproduction.
Fossil agnathans
 Although a minor element of modern marine fauna, Agnatha
were prominent among the early fish in the early Paleozoic.
 Two types of Early Cambrian animal apparently having fins,
vertebrate musculature, and gills are known from the early
Cambrian Maotianshan shales of China: Haikouichthys and
Myllokunmingia. They have been tentatively assigned to
Agnatha by Janvier. A third possible agnathid from the same
region is Haikouella. A possible agnathid that has not been
formally described was reported by Simonetti from the
Middle Cambrian Burgess Shale of British Columbia.
 Many Ordovician, Silurian, and Devonian agnathans were
armored with heavy bony-spiky plates.
 Ostracoderms – first armored agnathans, precursors to the
bony fish and hence to the tetrapods (including humans)—are
known from the middle sOrdovician, and by the Late Silurian the
agnathans had reached the high point of their evolution.
Most of the ostraoderms, such as thelodonts, osteostracans,
and galeaspids, were more closely related to the gnathostomes
than to the surviving agnathans, known as cyclostomes.
Cyclostomes apparently split from other
agnathans before the evolution of dentine
and bone, which are present in many fossil
agnathans, including conodonts. Agnathans
declined in the Devonian and never recovered