ChordataI Revised 1

8/13/2019 ChordataI Revised 1

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Phylum Chordata

The chordates are a group of particularinterest to us as we belong to it, beingmembers of the subphylum Vertebrata.

The chordates include all of thevertebrates (fish, amphibians, reptiles,

mammals and birds), but also two non-vertebrate subphyla: the Urochordata andthe Cephalochordata.


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Phylum Chordata

The chordates were in the 19thcenturyconsidered to have been derived fromprotostome ancestors (the annelid, mollusc,arthropod group).

However, a better understanding of embryologyshows that chordates are deuterostomes and

the invertebrates they are most closely related toare the Echinodermata (sea stars, sand dollars,sea urchins) and the Hemichordata (acornworms).


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Important differences in development

between protostomes and deuterostomes

The differences in development that distinguish

the protostomes and deuterostomes include:

Whether cleavage of cells in the early zygote is spiral

or radial. Whether or not, if the early blastomere is separated,

each cell can develop into a normal larva or not.

Whether the blastopore ultimately forms the mouth or

anus of the organism. Whether or not the organism possesses a coelom

and how that coelom is formed.


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Protostomes and Deuterostomes

Protostomes include the annelids,

mollusks, and arthropods.

Deuterostomes include the echinoderms

and Chordates, which includes the

vertebrates.


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Figure 23.02


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Characteristics of the Chordata

Chordates are:

bilaterally symmetrical

triploblastic

have a well developed coelom

have a complete digestive system


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Germ layers of a triploblastic

organism Endoderm: innermost germ layer of an embryo. Forms

the gut, liver, pancreas.

Ectoderm: Outer layer of cells in early embryo.

Surrounds the blastocoel. Forms outer epithelium ofbody and nervous system.

Mesoderm: Third germ layer formed in gastrula between

ectoderm and endoderm. The coelomforms in themesoderm. Mesoderm gives rise to connective tissue,muscle, urogenital and vascular systems andperitoneum.


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Coeloms

The coelom is a cavity entirely surroundedby mesoderm.

A coelom provides a tube-within-a-tube

arrangement which has many advantages:


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Advantages of a coelom

In mammals the pericardial, peritoneal, andpleural cavities are formed from the coelom.

Allows flexibility in arranging visceral organs

Permits greater size and complexity ofinternal organs as there is space for them togrow

Fluid-filled coelom can act as a hydrostatic

skeleton Cushions organs against impact

Allows organs to grow and moveindependently of your outer body wall.


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Five distinctive characteristics of

the chordates Five distinctive characteristics separate the

chordates from all other phyla: Notochord

Single, dorsal, tubular nerve cord

Pharyngeal pouches or slits

Endostyle

Postanal tail

Not all of these characteristics are apparent inadult organisms and may appear only in theembryonic or larval stages.


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Notochord

Notochord: the notochord is a flexible,

rodlike structure. It extends the length of

the body and is an anchor point for

muscles.

The notochord bends without shortening

so it permits the animal to undulate.

Figure 23 01


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Figure 23.01


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Notochord

In nonvertebrates and the jawless

vertebrates the notochord is present

throughout life.

However, in the jawed vertebrates it is

replaced by the vertebral column; the

remnants of the notochord being found in

the intervertebral disks.


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Single, dorsal, tubular nerve

cord In most invertebrates the nerve cord, if present,

is ventral to the gut.

In chordates, in contrast, the nerve cord is dorsalto the gut and notochord. The nerve cordpasses through the neural arches of thevertebrae, which protect it.

The nerve cord is enlarged in vertebrates into abrain, which is surrounded by a bony orcartilaginous cranium.


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Pharyngeal pouches and slits

Pharyngeal slits occur in aquatic chordates andlead from the pharyngeal cavity to the outside.

The pharyngeal slits are used as a filter feedingdevice in protochordates (i.e., Urochordata(Tunicates)) and Cephalochordata (lanceletse.g. Amphioxus).

Water containing food is drawn in through themouth by cilia and exits via the pharyngeal slitswhere the particles are trapped in mucus.

Figure 23 09b


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Figure 23.09b

Amphioxus


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In vertebrates the pharyngeal arches have

been modified into gills by the addition of a

rich blood supply and thin gas permeable

walls.

The contraction of muscles in the pharynx

drive water through the gills.


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In amniotes an opening may not form and

rather than slits only grooves called

pharyngeal pouches develop.

In tetrapods these pouches give rise

during development to a variety of

structures including the middle ear cavity,

eustachian tube, and tonsils.


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Endostyle or thyroid gland

The endostyle is found in protochordates

and in lamprey larvae. It is located on the

floor of the pharynx and secretes mucus,

which is used to trap particles.

The endostyle works with the pharyngeal

slits in filter feeding.


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Endostyle or thyroid gland

Some cells in the endostyle secrete

iodinated proteins and are homologous

with the iodinated-hormone secreting

thyroid gland, which is found in adultlampreys and vertebrates.


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Postanal tail

The postanal tail, some musculataure and thenotochord enable larval tunicates andamphioxus to swim.

The postanal tail evolved to allow organisms toswim and its efficiency has been enhanced bythe addition of fins. The postanal tail is present

only in vestigial form in humans (the coccyx)although tails as a whole are widespread amongvertebrates.

Figure 23.09b


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Amphioxus


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Classification of the Chordata

There are three subphyla in the Chordata:

Subphylum Urochordata: tunicates

Subphylum Cephalochordata: lancelets

Subphylum Vertebrata: fish, amphibians,

reptiles, birds, mammals, etc.


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Subphylum Urochordata

The Urochordata (tunicates named for the

tough tunic that surrounds the adult) look like

most unpromising candidates to be chordates

and relatives of the vertebrates.

The largest group, the ascidians or sea squirts

(Class Ascidiacea) as adults are marine, sessile,

filter feeding organisms that live either solitarily

or in colonies.
http://www.habitas.org.uk/marinelife/photo.asp?item=cioint


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Ciona intestinalis

(a solitary sea squirt)

0147.jpg
http://www.habitas.org.uk/marinelife/photo.asp?item=ciointhttp://www.habitas.org.uk/marinelife/photo.asp?item=ciointhttp://www.habitas.org.uk/marinelife/photo.asp?item=cioint


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http://www.habitas.org.uk/marinelife/photo.asp?item=synpul


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Synoicum pulmonaria a colonial sea squirt
http://www.habitas.org.uk/marinelife/photo.asp?item=synpulhttp://www.habitas.org.uk/marinelife/photo.asp?item=synpulhttp://www.habitas.org.uk/marinelife/photo.asp?item=synpulhttp://www.habitas.org.uk/marinelife/photo.asp?item=synpul


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Ascidians

Adult ascidians lack a notochord and there

is only a single ganglion in place of the

dorsal nerve cord.

Of the five characteristics of chordates

adults possess only two: pharyngeal gill

slits and an endostyle, both of which theyuse in filter feeding.


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Ascidians

The adult sea squirt draws water in

through an incurrent siphon and pushes it

back out an excurrent one.

Food particles are filtered out in the

pharyngeal slits with mucus from the

endostyle used to trap particles.

Figure 23.04


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15.4


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http://seapeachbio.blogspot.com/2012/06/ascidian-filter-feeding-mechanism.html

Ascidian filter feeding process. Water is drawn

through the animal by the beating of cilia.

Cilia lining the slits in the branchial sac or pharynx

Branchial sac orpharynx


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Larval Ascidian

Even though the adult ascidian hardly

resembles a chordate its larva does.

Larval ascidians are very small and

tadpole-like and possess all five chordate

characteristics previously outlined.


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Young larval ascidian


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Larval Ascidian

The larval ascidians role is to disperse andto achieve this it is free swimming.However, it has only a short larval life

(minutes to a couple of days) and does notfeed during this time.

Instead it searches for a place to settleand then attaches and metamorphosesinto an adult.


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Ascidian metamorphosis

During metamorphosis the notochord

disappears, the nerve cord is reduced to a

single nerve ganglion and a couple of

nerves.

Figure 23.06


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15.5


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Other Urochordate classes

Besides the ascidians there are two otherclasses of the Urochordata: the Larvaceaand Thaliacea.

Both are small, transparent planktonicforms. Thaliaceans are cylindrical or

spindle shaped whereas larvaceans aretadpolelike and resemble an ascidianlarva.

G f


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Garstangs hypothesis of chordate

larval evolution

In the 1920s it was proposed that the

vertebrates were derived from an

ancestral ascidian that retained its

characteristics into adulthood (the processby which juvenile characteristics are

retained into adulthood is referred to as

paedomorphosis).

Figure 23.12


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G t h th i f h d t


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Garstangs hypothesis of chordate

larval evolution

Garstangs hypothesis is supported by

embryological evidence, but more recently

molecular analyses have suggested that

sessile ascidians are a derived form andthat the free-living larvaceans are more

likely to be the closest relatives of the

chordates.


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Subphylum Cephalochordata

The cephalochordates are the lancelets, whichare small (3-7 cm long) laterally compressedfishlike animals that inhabit sandy sediments ofcoastal waters. They lack a distinct head and

have no cranium.

They are commonly referred to as Amphioxus asthis was the original genus name. There are 29species, five of which occur in North Americancoastal waters.

Figure 23.09b


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Amphioxus

Amphioxus is a filter feeder.

Water enters the mouth and then passes

through the pharyngeal slits, where food is

trapped in mucus. Cilia then move the

food to the gut.

Figure 23.09a


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Amphioxus


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Amphioxus

Amphioxus is interesting because it displays the

basic chordate characteristics in a simple and

obvious form because of its transparency.

Amphioxus is considered to be the closest living

relative of the vertebrates because it shares

several characteristics with vertebrates that

Urochordates do not possess.

Amphioxus characteristics


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Amphioxus characteristics

shared with vertebratesAs well as possessing the chordate

characteristics, amphioxus shares a

number of traits with vertebrates including:

Segmented myomeres (muscle blocks)

Dorsal and ventral aortas

Branchial (gill) arches (blood vessels running

over the gills).

Characteristics of vertebrates


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notpossessed by AmphioxusAmphioxus however lacks several

characteristics that biologists think the

ancestor of vertebrates possessed. These

include: Tripartite brain(with forebrain, midbrain and

hindbrain) protected by a cranium

Chambered heart Muscular gut and pharynx

List continues on next slide



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notpossessed by Amphioxus Various special sensory organs(eyes,

chemical and pressure receptors)

Neural crest(ectodermal cells that are found

on the embryonic neural tube and areengaged in the formation of the cranium, tooth

dentine, some endocrine glands and

Schwann cells, which provide myelin

insulation to nerve cells).

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ChordataI Revised 1