Superclass Agnatha

5/22/2018 Superclass Agnatha

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SUPERCLASS

AGNATHAWritten report

Submitted by:

Group # 1

BS Biology III-2

Abuan, Mary Jude Marby Faith

(evolution & physical features)

Aguila, Karlo Fernando

(Skeletal & muscular systems)

Alday, Addie Deanne


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(Circulatory, respiratory & digestive systems)

Asejo, Arra

(Urogenital systems)

Basa, Marvin

(Nervous & endocrine system)

Submitted to:

Prof. Fe Corazon A. Jacinto

Kingdom Animalia

Phylum Chordata

Subphylum Vertebrata

Superclass Agnatha

Class Myxini

*Class Pteraspidomorpha

Class Cephalaspidomorpha


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Order Petromyzontiformes

*Order Cephalaspidiformes*Order Anaspidiformes

*extinct

Evolution

(Photo credits: earthlife.net/fish.evolution)

From the words ameaning none, and gnath meaning jaw, the Agnathans are a

superclass of jawless fishes that existed 550 million years ago, in the middle Cambrian

period, and are the ancestors of the modern day vertebrates. The Agnathans consists of two

groups: the extinct Ostracoderms belonging to order Cephalaspidiformes; and the extant

Lampreys (Order Petromyzontiformes) and Hagfishes (Class Myxini) collectively called


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cyclostomes. The two Chinese fossils Haikouichthys and Myllokunmingia newly described

may represent the basal vertebrates.

1.A fossil of Haikouella, an Agnathan unearthed in

Chenjiang, China, from the Cambrian period.

(fossilmall.com) 2.Modern day lamprey

(antievolution.com)3.Chordate ancestor

(www.mhhe.com/biosci)4.Hagfish showing its capability

to convolute itself.

2

1
http://www.mhhe.com/bioscihttp://www.mhhe.com/bioscihttp://www.mhhe.com/bioscihttp://www.mhhe.com/biosci


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(Photo credits: http://planet.uwc.ac.za/

Ostracoderms

These are oldest known craniates that date back

to the Ordovician period. They are fishes with a

heavy dermal armor plates where it is largest on

the head forming a shield. They lack paired finsand have only a median fin down the midline of

their back that provides stability in locomotion.

The head skeleton of ostracoderms have four

dorsal apertures- two for a pair of upward-facing

eyes, an aperture for a median pineal eye, and an

opening for a single naris. The head contained

cartilage.

The ostracoderms disappeared after the true

fishes appeared on the Devonian period.

Physical Features of Living Agnathans

Aside from the lack of jaws, the Agnathans are characterized by:

Absence of paired fins;

Presence of several gill pouches;

The gills open through the pores rather than on slits

One (hagfish) or two (lamprey) semicircular ducts;

The presence of notochord (which makes them distinctly vertebrates) in both larvae and

adults which serves as their sole axial skeleton throughout their life;

A single nostril;

Buccal funnel and rasping tongue-like extension

Slimy skin, and caudal tail


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(photo credits: http://www.glogster.com/siryapsalot/agnatha/)

Photo credits: (http://www.always-

drunk.com/2014/05/hagfish-part-2.htm

Also, they do not have

identifiable stomach or

appendages and are

ectothermic, meaning

they can change theirbody temperature to

match with their

environment. They

possess a 2-

chambered heart.

Gone are the body

dermal armors that

their co-agnathans had

and in its place, a soft,slimy eel-like body.

Myxini (Hagfish)

The hagfishes are living marine agnathans that have elongated,

eel-like bodies, and one continuous fin fold on the back and

around the tail. They have a cartilaginous skull, one semicircular

ducts, absence of eye musculature, a single olfactory capsule with

few folds in the sensory epithelium. Their mouths, located dorsally,

are shallow buccal funnels that lack rasping denticles, and have

two pairs of horny, comb-shaped teeth on a cartilaginous plate

that protracts and pulls. These teeth are used to grab and pull food

toward the pharynx. They are chiefly bottom-feeding scavengers.

Their gills are located in pouches on one side of the body ranging

five to fifteen p Hagfishes can radiate large amounts of mucus and

the typical species is Myxine glutinosa. There are 42 living speciesof hagfishes.


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Photocredits: (http://en.wikipedia.org/wiki/Lamprey#mediaviewer/Filea:Lamprey_anatomy.png)

Lamprey

Lampreys are a group of eel-like jawless fishes found in temperate rivers and coastal seas, some

species live in fresh water. They may have on or more dorsal fins separate from the caudal fin.Others, like the sea lamprey (Petromyzon marinus) is anadromous- they are hatched and grow

in fresh water, migrate to the ocean to mature, and return to fresh water to spawn.


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Unlike hagfishes that only have a

skull, lampreys have a complete

braincase and rudimentary true

vertebrae. Adult lampreys have large

functional eyes, one nostril on top ofthe head, and seven gill pores on

each side of the head. Lampreys

have a round, suckerlike buccal

cavity, on the ventral portion, filled

with rows of horny denticles and a

cartilaginous rod of rasplike-tongue.

An internal ring of cartilage also

supports the rim of the mouth.

Some species are parasitic to fishesand fasten onto those, sucking into

their flesh, and some are filter feeders.

Circulatory System

An Agnathan Circulatory system represents the circulatory system that is used by Vertebrates

that are not based on mouth breathing. A key example is the circulatory system of fish. Themost common fish are lamprey and hag fish.

The common fish circulatory system works as follow: In the lamprey heart the atrium and the

ventricle are side by side. The sinus venosus entering the atrium laterally. The non-muscular

values prevent the backflow of blood. There is no separate coronary blood supply, and the

heart must obtain its oxygen from the blood as it goes through. The arterial system in

agnathans is most obviously modified because there are more than six sets of gills. Eight

branches emerge from the ventral aorta, which splits into two, unlike the single vessel in most

vertebrates with gill slits. Oxygenated blood from the gills is then collected into eight efferent

vessels, which join to form a dorsal aorta, single for most of its length. Internal carotid arteries

arise from the dorsal aorta, but the ventral part of the head is supplied from anterior efferent

branchial (gill) vessels, not from the anterior part of the ventral aorta. The venous system does

not include a renal portal section, and there is asymmetry of the common cardinal veins, which

take blood from the dorsal anterior and posterior cardinal veins down to the ventral heart. In

embryos there are two of these, one on each side of the body; in lampreys, the left one

Photocredits:

http://en.wikipedia.org/wiki/Lamprey#mediaviewer/File:Sea_Lamprey_fish.jpg


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disappears during development, while in hagfishes the right one disappears. Hagfishes also

have accessory hearts in the venous system at several points. No other vertebrate has these

structures.

It is not clear why there is such drastic differences between the circulatory systems of the basic

vertebrates and the Agnatha. There is some history of the animal that needs to be remembered.

The Agnatha is a decendant of other animals.

Digestive System

Agnatha, common name "jawless fishes", is a superclass of jawless fish in the phylum Chordata,subphylum Vertebrata. The group excludes all vertebrates with jaws. Most Agnatha have simple

to complex digestive systems, and they can range in size from just a few centimeters up to 2

meters. They can be found with both straight digestive tubes as well as coiled tubes with

lengthened tubes.

Representative examples of Agnatha include the sea lamprey and the hagfish. General features

of Agnatha include a lack of jaws, unpaired fines, eel-like bodies, notochord that persists into

adulthood, seven external gill openings and a skeleton that is composed of cartilage. They

typically have no scales and smooth, shiny skin.

Agnathans are ectothermic, meaning they do not regulate their own body temperature.

Agnathan metabolism is slow in cold water, and therefore they do not have to eat very much.

They have no distinct stomach, and instead they have a rather long gut, more or less

homogenous throughout its length. Lampreys are parasitic, feeding off 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. Agnatha breathe using a system of gills.


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Movement typically can be observed to be in a rippling, fluid manner. One common Agnathan is

the Hagfish. Hagfish attach themselves to dead or dying fish with their barbell adorned, cup-like

mouth and use a toothed tongue to scrape a hole into the fish. They can also burrow into the

body cavity. They are also known to tie themselves into a knot and work the knot downward.

They then leverage themselves against their food item in order to tear off flesh. Similar feedingpatterns can be observed across the Agnathan superclass as a whole.

Digestive-respiratory System

The digestive and respiratory systems are closely related anatomically. The buccal funnel

continues above the tongue as the narrower buccal cavity. At its posterior end, the buccal

cavity divides into a smaller dorsal tube (esophagus) and a larger ventral tube (pharynx).

There are six openings in the walls of the pharynx. These are the internal gill slits, whch lead

into the gill pouches. Each pouch is lined with leaf-like structures, the gill lamellae. The

lamprey respires by pumping water in and out of the gill pouches through the openings at the

surface (external gill slits).


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Excretory System

Species found under this taxon have an excretory system that mainly relies on kidneys. Thesekidneys do not only send wastes out of their body, but they also play a key role in keeping the

amount of salt regulated inside the body.

Class Myxini (Hagfishes)

Hagfishes have mesonephric kidneys as their main organs for waste management in their body,

but in their larval stage, their kidneys are archinephric of type.

Archinephros is said to be the ancestral condition of vertebrate kidney. The Wolffian

ducts extend between the body cavity to the back that leads to the exterior. A series of

tubules, one pair for each body segment, connects the body cavity with the Wolffian

ducts. Each tubule is ciliated where it opens into the body cavity, and a knot of

capillaries occurs at each of these openings, which are called nephrostomes.


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Figure A.2 Pronephros

Then in the adult stage of hagfishes, pronephros becomes their functional kidney, the

simpliest kind of excretory organ. Pronephros develops from the intermediate

mesoderm. It consists of of paired nephrons, nephrostomes made from epithelial cells

that is conjoined laterally by pronephric ducts


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Class Cephalaspidomorphi (Lampreys)

In adult lampreys, opisthonephros are their functional kidney. But in their larval stage,

ammocoetes, pronephros is present as their excretory organ.

Opistonephros of the lamprey consists of a long strap-shaped body on each side without

peritoneal connections. The kidneys lie on the either side of the mid-dorsal line, from

which each is suspended by a mesentery-like membrane.


14/18Figure A.5 Ventral view of coelomicviscera of female lamprey.

Figure A.4 Petromyzon marinus

In Petromyzon marinus a vestige part of the archinephric

duct, which was associated with the degeneratepronephros, extends forward from the opisthonephros.

The ducts from the 2 sides unite posteriorly to open into a

urogenital sinus which leads to the outside through an

aperture at the tip of a small urogenital papilla. Two

slitlike opening, the genital pores, connect the urogenital

sinus with the coelom.

Reproductive System

Gonads are usually paired, but in cyclostomes, one

gonad is only present as a result of either fusion of

paired structures or of unilateral degeneration.

Gonads are derivatives of the mesoderm. In the

very early stages of development, sexual


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differences are very difficult to recognize. The ovaries and testes are attached to the

dorsal body wall by mesentery-like bands of tissue, the mesorchiumin the male and the

mesovarium in the female.

The lampreys, sexes are separate in species. The adult female lamprey has a single gonad,representing a fusion of two, which runs the lengths of the body cavity and is attached to the

mid-dorsal body wall by a single mesovarium. At the height of breeding season, the ovary

occupies the greater part of the abdominal cavity. In ammocoetes stage, two ovaries are

present, but they later fuse into a single organ.

The hagfish is hermaphroditic. The anterior part of the single gonad being ovarian in nature and

the posterior portion is testicular. Usually one or the other region matures and becomes

predominant. Only the gonad that is left develops fully.

Nervous system

General characteristics:

- Brain is poorly developed

- Notochord persists throughout its lifetime

- consist of a slender spinal cord and the brain (enlarged, lobed structure just anterior to the tip

of the notochord)


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- The nerve fibers dont have a myelin sheath (which can be found on more complex vertebrates)

which makes the nervous conduction slow.

- There is a light sensitive pineal eye (homologous to the pineal gland in mammals)

Characteristics specific in Hagfishes

-has keenly developed sense of smell and touch but poor (or degenerated) eyesight

- Dorsal nerve cord with differentiated brain without cerebellum

- 10 pairs of cranial nerves

- Dorsal and ventral nerve roots united

- have four pairs of sensing tentacles arranged around their mouth

Characteristics specific in Lampreys

- Dorsal nerve cord with differentiated brain, small cerebellum present

- 10 pairs cranial nerves

- Dorsal and ventral nerve roots separated

- Has olfactory sac (dark mass of tissue antero-dorsal to the brain)

- Axons are particularly large


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Endocrine System

Characteristics specific in Hagfishes

- known for its ability to produce large amounts of slime from its copious glands on itsbody covering; uses it as defense mechanism from predators


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- Body fluids are isoosmotic with saltwater (slime from copius glands form white milky

substance when mixed with saltwater).

Characteristics specific in Lampreys

- Body fluids osmotically and ionically regulated

- Possess oral or pharyngeal glands (secretes an anticoagulant fluid that disables its

prey's blood from clotting, and at the same time, breaking down its tissues for an

effective feeding process)

Documents

Superclass Agnatha