Chordata- Two Invertebrate Subphyla: Urochordata and Cephalochordata

by Katey Duchin


Introduction: There are four defining features that characterize the phylum Chordata.
  • Notochord- The notochord, which is present in all chordate embryos, is a skeletal rod that is found between the digestive tube and the nerve chord. It supports the body of the organism. [It is composed of cells derived from the mesoderm and defines the primitve axis of the embryo. RS]
  • Dorsal, Hollow Nerve Chord-The nerve chord is located dorsal (behind) the notochord. Unlike the solid nerve chords of other phyla, the chordates have a hollow nerve chord.
  • Pharyngeal Slits- The pharynx, which is the area behind the mouth, has many slits. They are used as suspension-feeding devices in many invertebrates, which means that they are used to capture food particles from water.
  • Muscular, Postanal Tail- This part of the body is made up of muscles and bones. The tail is located behind the anus.

The two subphyla of invertebrates are the urochordates, known as tunicates, and the cephalochordates, known as lancelets. Tunicate larva have the four characteristic structures of the chordates, however the "U" shaped adults do not have a notochord, dorsal, or tail. On the other hand, all lancelets have all four of the characteristic features of Chordata. Lancelets are very small organisms; they are only a few centimeters long.

In most vertebrates, the embryo's notochord is eventually replaced with bony vertebrae or cartilangenous tissue; in these two subphyla, however, the notochord is retained through adulthood. Organisms of these subphyla are probably the vertebrates' closest living relatives. (SF)

Acquiring and Digesting Food

Urochordates- Water enters the organism through an area known as the incurrent siphon. It then goes through the pharyngeal slits into the atrium, which is a chamber. The seawater exits via an opening called the excurrent siphon. A mucus net filters the food, which the cilia (hair-like structures that move particles and fluids) pass to the intestine, the area where digestion begins.

external image 34-03-Tunicate-L.jpg

Cephalochordates- Cephalochordates take water in via the mouth. The water is drawn in by the cilia producing beating movements. The cilia are located on the wheel organ, which is a set of folds inside the mouth. The oral cirri, projections around the mouth, cleanse the water, and then the water passes through the gills.
There is a mucus net covering the pharyngeal slits. This net traps little food particles that are available in the seawater. The food passes through the digestive tube. The filtered water passes through the slits to the atrium, and out of the organism through the atriopore. The remainder of the digestive tract is basic: the hepatic caecum squirts digestive enzymes and the rest of the digestive process in carried out in the iliocolinic ring which is part of the intestine. (AK)

Sensing the Environment

The lancelets have sensory tentacles, called oral cirri, around the mouth area. Both the urochordates and the cephalochordates do not have highly developed sensory organs.
Invertebrates in general have very simplistic nervous systems. One of the most simple that some invertebrates have is called nerve-net, where the organism is able to sense some environmental changes such as light or touch. (JAF) There is a nerve chord above the notochord in cephalochordate, but it doesn't have a brain or eyes. (JL)
In this picture of a lancelet (belonging to subphlyum Cephalochordata), we can see the oral cirri around the mouth. KNS


Urochordates- The larvae are able to swim. They attach themselves to a surface (ex. a rock, boat, or dock) and go through metamorphosis. During this process they lose their notochord, tail, and nerve chord. Most adults are sessile, meaning permanently attached to something and immobile.
Cephalocordates- Lancelets have muscle segments which cause their sinusoidal (side-to-side) swimming. The contraction of the muscles causes their notochord to flex. These muscles are called myomeres. They are clumped together and are aligned like two Vs stacked over each other. (Walker Keenan).


Urochordates- Gas exchange occurs across the external surface of
the body. The pharynx allows for water to enter the branchial basket which contains the mucas covered gills (MSR)
Cephalacordates- The majority of the gas exchange occurs across the external surface. The gill slits and the pharynx play a minor role in respiration.

Metabolic Waste Removal

Both of the subphyla have no kidney. In urochordates, the food from the intestine empties into the anus, which exists the organism via the excurrent siphon. In cephalochordates, the food passes from the intestine and exits the organism through the anus.


The urochordates have an open circulatory system, meaning that the blood is not contained in arteries or veins. Most of a tunicate's organs (except for the heart, pharynx, and gonads) are located in a membrane called the epicardium, surrounded by a jelly-like matrix called mesenchyme (JM). Lancelets have a closed circulatory system, but do not have blood cells or a central heart. Cephalochordates' circulatory system consists of a ventral vessel and a dorsal vessel that carry colorless blood. (APS)

Self Protection

Urochordates- Tunicates are also called sea squirts because when they are disturbed they shoot out water through their excurrent siphon.
Cephalochordates- One characteristic quality of the lancelets is that they tend to burrow in the sand and leave only their anterior end exposed. When disturbed, they swim to a new location. Their transparent body helps protects them against predators.

[-In this video some Urochordates shoot out water as they are disturbed. (DG)]

external image v44n1-carman4n_8846.jpg
urochordate defense (LD)

Temperature Balance and Osmotic Balance

Their body temperature is dependent on the environment.
The osmotic balance is dependent on the seawater that surrounds them

Classes of Urochordates

1.Class Ascidiacea-Filter feeders that are best known as "Sea Squirts". They are sessile (not free moving) and live in the intertidal zone. They filter out water for plankton
2.Class Thaliacea-Oddly shaped barrel-like organisms best known as "salps". Salps use jet propulsion to filter feed by contracting their muscles, causing their body to pulsate. Salps are free-floating.
A string of salp captured in the Red Sea
A string of salp captured in the Red Sea

These salps are sound in the Red Sea.
3.Class Larvacea-The most specialized Urochordate that builds a thin-walled mobile home type structure that funnels water throughout their bodies. (Jake Schwartz)

a  look at deuterostomes and the evolution of vertebrates (BY)
a look at deuterostomes and the evolution of vertebrates (BY)

Review Questions:
1) Describe the differences between Urochordates and Cephalochordates in terms of: acquiring and digesting food, respiration, and locomotion. (SW)
2) What are the four defining features that characterize the phylum Chordata? [Rabya Saraf]
3) How do Chordata-Inverterbrate sense their environment? Give an example of a specific organism. (LS)
4) Discuss the differences and advantages of an open vs closed circulatory system. Use uruchordates and lancelets to illustrate your point. (AG)
5) How do Cephalochordates acquire and digest food? (JJF)
6) Their homeostasis is dependent upon what? (AS)
7) How does the "nerve net" compare to ganglia or brain in other organisms? (DRM)

Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.

Tunicate picture
Lancelet picture
Jurd, Richard David. Animal BIology. Second Edition. Garland science publishers, 2004. (Sarah Fleming) (Jake Schwartz) (Ali Kirsch) (JAF) (KNS) (Jackson Murphy) [BY] K.) (Alyssa Zisk) (
APS) (Liz Daley) (Jesse Landy) (DG)