Urochordates and Cephalochordates
Zachary Jay

external image a_urochordate.gif
Diagnostic Characteristics

Anatomical features of Chordate Invertebrates
There are four key anatomical characteristics that are apparent in all Chordates. Among these are the notochord, hollow nerve chord, pharyngeal slits, and postanal tail. It is important to understand these four characteristics as invertebrate chordates use these various components for complex functions varying from obtaining food to locomotion.

Anatomy of Tunicate
Anatomy of Tunicate


Present in all chordate embryos and some adult chordates the notochord is a flexible tube that provides skeletal support for the organism. Running in a longitudinal direction, the notochord consists of fluid filled cells encased by a layer of fibrous tissue. The notochord has two main uses. First, it’s ability to act as a flexible rod makes it useful for the motion of swimming. In vertebrates, the notochord appears very early on in embryonic development and releases chemicals triggering the thickening of the ectoderm, eventually becoming the spinal cord and brain.

Dorsal, Hollow Nerve Chord
Developing from the ectoderm, the nerve chord of a chordate is a hollow tube located dorsal to the notochord.
The nerve cord being hollow means that it is a canal filled with fluid, unlike the solid nerve cord of invertebrates. (PS Source 12)

Pharyngeal Slits
Located on the pharynx, the pharyngeal slits open and close to allow water that enters the mouth to exit without traveling through the digestive system.
Between the mouth and the pharynx, this slit is a feature of all chordates. Throughout evolution this feature has been modified as new species developed. In primitive chordates such as urochordates and cephalochordates, this opening is used to filter food from the water. Through it the ingested water can be forced out before entering the digestive system. In more adapted chordates, the pharyngeal slits have been modified for gills and are used in gas exchange with the water. Once certain escaped the restrictions of the water the pharyngeal slits became unnecessary and are mostly only present during the embryonic stage of development. (MS 14)

Muscular, Postanal Tail
Instrumental for most aquatic species, the postanal tail of a chordate contains skeletal like structures.

Urochordates and Cephalochordates
A Urochordate which is also known as a tunicate or sea squirt is a marine animal. There are three categories of tunicates- Some tunicates may be found attached to rocks, docks, and boats. However, other tunicates such as plankton do not adhere to surfaces. A third category of tunicates live in colonies. An adult tunicate barely resembles a chordate as it does not posses a notochord, nerve chord, or tail. It does however posses pharyngeal slits. The reason that Urochordates are considered to be chordates is that some species have larva that posses all four anatomical features.

A Cephalochordate which is also known as a lancelet is a marine organism that is found in huge densities at the bottom of sea and costal regions. Unlike tunicates, Both adult and larval lancelets have all four anatomical features and do not undergo a metamorphosis.

While the two phyla have many differences one commonality between the two is that the organisms of both phyla are invertebrates. This means that both Urochordates and Cephalochordates have no backbone.

Urochordates: They have notochord-like structure in the tail. As a larva, the urochordate has all the chordate characters, but lose a lot of characteristics as an adult. The adult organisms only have a slitted pharynx and a ventral heart. The pharynx is part of the throat posterior, behind, to the mouth.The adults are usually sessile, non-moving, or plankton filter-feeders. (AR)

Cephalochordates: They have notochord-like structure in head. This organism is similar to ancestors of Craniates. Craniates are chordates that have undergone more complex cephalization having a head. Cephalochordates are usually suspension feeder in the benthic, lowest level of body of water, zone of oceans. There are only few species of cephalochordates, but are mostly plentiful in their ecosystem, (AR)

(Source 5 and 6)

Acquiring and Digesting Food

Both Tunicates and Lancelets attain their food through similar methods and are known as suspension feeders.

The figure above shows the varying anatomies of the Tunicate larva and adult

When water enters the tunicate through the incurrent siphon, it passes through the pharyngeal slits and into the atrium. A mucous net filters the food from the water, and the food is passed into the intestine of the tunicate. When digested, the food particles exit the anus and leave through the excurrent siphon of the tunicate.
Tunicate's pharynx is covered by tiny hairs (ciliate cells) that passes consumed food go through the oesophagus. The digestive system is U-shaped, the anus emptying directly to the outside.
Tunicates are filter feeders(MP-9).

Anatomy of a Lancelet

When water is drawn into the mouth of a lancelet by ciliary pumping the mucus net across the pharyngeal slits traps tiny food particles. Water exits through the slits and the trapped food particles travel down the digestive tube. After digestion the food particles exit through the anus of the lancelet.

Sensing the Environment

Neither lancelets nor tunicates posses elaborate sensory organs.

Urochordates possess light and tilt sensors. Cephalochordates possess a notochord and nerve chord, but have not brain or any other specialized sensory organ. (AR) (source 4)


Both lancelets and larval tunicates propel themselves through the water using their muscular postanal tails. The segmented nature of the tails allows for a side to side movement which thrusts the lancelets and tunicates forward. Adult tunicates are unable to move.


The gill slits of the tunicates help with the process of gas exchange. Gas exchange also occurs across the external body surface of the tunicate.

Because lancelets have no respiratory system, most gas exchange occurs across the external body surface of the lancelet. The pharynx and gill slits of the lancelet also play a minor role in gas exchange.

Metabolic Waste Removal

The products digestion leaves the tunicate through the anus and excurrent siphon.

The products of digestion leave the lancelet through the anus.

While heartless, the overall pattern of the lancelet circulation is similar to that of the vertebrates. Blood is pumped by contractile blood vessels (the ventral aorta does most of the pumping) and by the gills' blood vessels. Blood goes from the back of the body to the ventral aorta, and then branches upward towards the gills' blood vessels. (SP)

Even though they do not have blood cells or a heart, Lancelets have a closed circulatory system. Cephalochordates' circulatory system have a ventral vessel and a dorsal vessel used to carry their colorless blood. (CP source 13)

Lancelet circulatory system (JS 15)
Lancelet circulatory system (JS 15)

The blood of cephalochordates, or lancelets, is devoid of pigment, hence the lack of color, and cells, functioning mostly to distribute nutrients and not to participate in gas exchange and transport. Instead, because they lack a respiratory system, gas exchange occurs at the body surface. (MR; Source 16)

Self Protection

The tunicate is known as a sea squirt as its form of defense is to squirt water when it is molested.

Tunicates possess a couple other methods of defense, however. Some can store in their tunic cells a high concentration of toxic compounds, like sulfuric acid. Others have sharp spines that protrude from their bodies. This can fend off, for example, a snail. (Matt B - Source 7)

The lancelet burrows its self in sand such that only the mouth and tentacles are visible.

Osmotic Balance

Both tunicates and lancelets are osmoconfomers. An osmoconformer is an organism that does not continually adjust its internal osmolarity and is therefore in sync with the osmolarity of its environment.
Osmoconformer conformity occurs when the organism cannot regulate or cannot keep solutes out of its bodily fluids when it is at a concentration that is different from the medium it is surrounded by. The organism is therefore considered to be isoosmotic when the solutes of the organisms bodily fluid are the same concentration as the ones outside of it. Being isoosmotic means that when the organism is at osmotic equilibrium with the external medium it will not gain or lose solutes or water. (IL - source 11)

Temperature Balance

Both tunicates and lancelets are ectotherms. An ectotherm is an organism that does not generate enough heat to impact its body temperature.

Lancelets have males and females that reproduce sexually. The sperm and egg are released at the same time in the water and fertilization occurs creating a gamete. Once 12-15 pairs of gills have formed, the gametes sink to the ocean floor and officially transform into young lancelets. (AP-source 8)

Tunicates reproduce in two ways. One way is by budding, which results in colonies of sea squirts. These colonies are joined at their bases by slender stalks or are embedded in slabs of common tunic material. Tunicates can also reproduce sexually since they are hermaphroditic (individual organism has the reproductive organs of both sexes). These free-swimming larva are often called tadpoles and have the characteristic features of chordates also found in the embryos of vertebrates: a dorsal, hollow nerve cord,a notochord, and gill slits leading into the pharynx. Eventually, the tadpole undergoes metamorphosis into its adult form (VN).

Review Questions

1. Why have chordates been selected for the inclusion of the notocord? (RK)
2. What is the difference between urochordates and cephalochordates? (MM)
3. what is the major difference between the invertabates nerve cords and a chordates?(RJ)
4. What are the two different methods of tunicate reproduction? (JP)
5. How is suspension feeding similar to how invertebrates respire through gills? (LPE)
6. What are the four structures that all chordates possess and how do each function? (RW)

Closest vertebrate ancestor: Fish
Closest invertebrate ancestor: Echinoderm

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