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