Urochordata & Cephalochordata

Urochordata & Cephalochordata(Tunicates & Lancelets)

Urochordata

Commonly referred to as: Tunicates, Sea Squirts. Salps and Larvaceans

Live in marine environments

About 2000 known Species

Filter feeders

No excretory system

Body enclosed in a tunic made of secreted proteins and something similar of cellulose

Comes in variety of colors; most are translucent, or whitish, can come in colors of Red, Brown, Yellow or Blue

Ascidiacea Ascidiacea, Thaliacea, Larvacea

Are commonly referred to as Sea Squirts of Tunicates

Are the biggest group within the Urochordata

Coastal animals, live in 400 meter depth, but have been seen in 5 000 meter depth

Solitary or Colonial

Colonial share a common exhalent siphon

Tunic is secreted by the epidermis

Can shoot a jet of water to ward off predators

Ascidiacea Body

All organs except the pharynx are enclosed in a membrane called epicardium and surrounded by mesenchyme (a jelly like substance)

This is called visceral cavity

The Atrial Cavity is bigger then the Visceral Cavity, and cointains the enlarged pharynx

The Pharynx has many small holes where the water passes through

The Pharynx is connected to the digestive and inhalent siphon

The Atrialm Cavity has two sets of Cilia

One set moves the water from the inhalant to the exhalent siphon

The other moves the mucus lining in the pharynx

Feeding

The Tunicates feed by siphoning water through the pharynx

When the water passes through the holes in the pharynx, food particles get stuck on the mucus coating of the pharynx

The mucus gets moved down the pharynx towards the digestive system

The mucus is secreted by special cells

Blood System

Blood is pumped through small spaces in the mesenchyme

The spaces are called sinuses, they are not true blood vessels

The heart is very different then most

It beast around ~100 times in one direction

Stops for a bit

Then beats for around ~ 100 times in the other direction

The blood is a clear in color

The blood often contains high concentrations of vanadium

Vanadium is a rare element, and is found sparely in sea water

No one hasfigured out why the Tunicates collect this element

Reproduction

Tunicates are generally hermaphroditic (both male and female)

Tunicates avoid self fertilization in two ways:

The sperm and eggs may be chemically designed to not accept each other

Or the sperm generally mature before the eggs

Eggs are retained within the body and the sperm are released into the sea

Eggs are fertilized by the incoming water

Once fertilized, the eggs stay inside the body until they hatch

Ascidiacea Larvae

It is more obvious that they are part of the chordate phylum

Do not feed, are more of a dispersal form

Live only a few hours

Attach themselves to the ocean floor using adhesive glands on their heads

It then begins a metamorphosis and looses some of the aspects that classify it as a chordate

Thaliacea

70 species

Live in warmer waters then the tunicates

They feed and swim, the Inhalant and Exhalent Siphons are on opposite ends, so it propels them

Three classes:

Pyrosomida

They live in colonies

Salpida & Doliolida

Are not colonial

Pyrosomida

When living in colonies, the colony is barrel shaped

The barrel has one open end

Each animal takes in its own water, gets expelled through common exit, also serves as propulsion (can become several meters long)

All are hermaphroditic in a colony, called gonozooids

They develop one egg at a time (fertilized internally)

develops into a oozoid (asexual form with a short life span)

Oozoid creates a new gonozooid, which is released into the water

This is called alternation of generation

Salpida (Salps)

Are Asexual dominant

1.5 to 19 cm long

Buds form around the asezual oozoid and develop into the gonozooids

When the gonozooids are released they stay linked and can do so untill the chain is very long (maximum of 2.5 m)

Each gonozooid develops only one egg and the oozoid hatches inside the parent, which means they are viviporous (they give birth to live offspring)

Doliolids

Average only 1 cm long

Once the oozoid form reaches a certain size it gives rise to 3 different buds, which form a temporary colony

Trophozooids (feed the temporary colony)

phorozooids (essentially Doliolids, have no gonads, support and carry the embryo gonozooids away from the parent oozoid)

Gonozooids (are the embryos)

Larvacea Sometimes called Apendicularians

Only 70 species known to man

Live on warm water surfaces

They have a oval trunk with a long thin tail

They retain their tail throughout their life, unlike the other Urochordata

Tail has muscle cells attached to it for swimming

Feed with a filtering system inside the house, eat much smaller particles then the other Urochordata (particles as small as 1 micron)

The house is the same color and density of the sea water, so it is very hard for us to see

The Larvacea abandons the house about every 4 hours, and secretes a new one

It is about 2.5 cm in diameter

Larvacea Reproduction

Only sexual reproduction

Most species are hermaphroditic, at least one is gonochoristic

Testies mature before the ovaries to prevent self fertilization

Sperm are released first, and the eggs (ova) later on

Eggs (ova) are released by rupturing the body, this means the animal dies

The larva of the Larvacea look like small versions of tunicate larva, though thy mature much faster

Cephalochordata

Also called Lancelets

There are about 28 species

5 to 10 cm long

All species are marine

Exhibit all the four basic characteristics of the chordata (a dorsal nerve cord, a notochord, a post annul tail and pharyngeal gill slits)

The lancelets have 3 body openings:

A mouth guarded by buccal cirri tentacles

A antriopore towards the tail where the water exits

A anus which is located behind the antriopore

Are filter feeders, most of their body is taken up by a enlarged pharynx

The way the Lancelets feed is very similar to the Tunicates

Blood System

Has a main ventral and paired dorsal aorta

No central heart, but a number of bronchial hearts

The bronchial hearts are located where each of the main vessel branches meet the aorta

The blood of the lancelets lack hemoglobin and is colorless

Cephalochordata Nerve System

Lancelets have a central nerve which is enclosed in a sheath of collagen fibres below the notochord

From the central nerve arise smaller ones that serve the rest of the body

Lancelets do not have a brain or cranium like the vertebrata

Lancelets have metameric muscles, meaning they partially overlap

These muscles are called myomeres

This structure is very common in fish

When the muscles contract on one side the tail moves because the notochord is stiff

Cephalochordata Reproduction

Lancelets are Gonochoristic

Sperm and Eggs are released into the water

The fertilized eggs hatch into larvae

The original larvae turn into amphioxis larvae, amphioxis larvae resemble the adult form more

Both larvae forms are palegic, they life in the water not on the ocean floor

Amphioxis larvae fee on plankton till they are large enough to metamorphosise into the adult form

Cephalochordata Evolution

Lancelets represent a very basic form of chordata life

Some scientists believe this is how the ancestors of chordata looked

Generally accepted that it is a side branch of evolution rather then a direct ancestor