Invertebrate Chordates: Urochordates and Cephalochordates
(Josh Czik)

34-04a-LanceletAnatomy-L.gif
A cephalochordate, commonly known as a lancelet. They are considered the idealized form of a chordate. (http://io.uwinnipeg.ca/~simmons/16cm05/1116/chordate.htm)


Introduction

The phylum chordata involves all vertebrates, including humans, but it also includes two lesser known subphyla: urochordates and cephalochordates. Although species in these subphyla are not vertebrates, animals with a backbone, they still include key features that vertebrates have. Invertebrate chordates are known to be the closest ancestors to vertebrates. The subphyla urochordates goes through a metamorphosis during its life. The first stage of an urochordates life is the tadpole stage, soon the urochordate will attach itself to a hard surface, lose its tail and ability to move, and with this its nervous system largely disintegrates. (12, GR) Urochordates are all marine and they are enclosed in a tough cellulose like tunic. The larva form posses more of the chordate characteristics than adults do such as gill slits, dorsal nerve cord, and notochord. The larva stage lasts only a few days. Adults are attached by the base, so they must obtain their food by trapping food particles. (15) (BMB)

A lancelet, a common type of cephalochordate, usually grows up to five centimeters long (SES 13). They are abundant in Asia and usually live beneath the sand floor of temperate and tropical waters (SES 13).
There are three major classes of urochordates: Ascidiacea, Thaliacea, and Larvacea. Ascidians are sessile organisms found in the intertidal zone. Commonly referred to as “sea squirts”, ascidians are all filter feeders. Some are solitary and others are colonial, and most ascidians are monoecious. Thaliaceans are barrel-shaped organisms that are open at either end, and are commonly referred to as “salps”. They are made up of circular muscle rings that draw water in one end and out of the other, allowing them means of propulsion and filter-feeding. Larvacians are the most specialized of the urochordates. They resemble ascidian larvae, and create a “mobile home” within which they live throughout their lives. (16 Nangia)
The urochordates are also called turnicates. Most of them are sessile (staying in one place). Cephalochordates are also known as lancelets, having all characteristics of an invertebrate chordate even in adulthood (1) (see below) [HZ].
Cephalochordata are commonly represented by the Amphioxus ("sharp at both ends"). Amphioxus are 2-3 inches and live on seashores. It is appears similar to a fish, but it lacks complex organs, or any other bony structure. Its primary feeding structures are concentrated at the anterior end. (15) (BMB)

A picture of an adult urochordate (KN) (10)
A picture of an adult urochordate (KN) (10)



Diagnostic Characteristics of Invertebrate Chordates

slide0010_image032.jpg
http://www4.alief.isd.tenet.edu/cahowe/AP%20Biology/powerpoints/PAK%2014%20Animals%2007.htm#slide0010.htm

Urochordates and cephalochordates have four key anatomical features that distinguish them as a part of the chordata phylum.
1. Notochord
The notochord is a long, flexible rod that runs along the back of the chordate. The inside of a notochord is typically filled with a sort of fluid, while the outside is relatively stiff. The notochord serves the skeletal support of the animal, as shown in the figure above (8 BL), but it does not have to be made of bone.
2. Nerve Cord
The nerve cord serves as the central nervous system of the chordate. It runs down the back of the animal like the notochord. An important diagnostic characteristic of the nerve cord is that unlike other nerve cords in non-chordates, it is hollow and not solid. The nerve cord also develops into the spinal cord and the brain of the chordata. All chordates have a dorsal, hollow nerve chord, an example of which is shown in the figure above (8 BL).
3. Pharyngeal Slits
Behind the mouth of an invertebrate chordate are the pharyngeal slits (gills), like those shown in the figure above (8 BL). These slits open on the outside of the animal and allow water to pass out of them instead of through the entire digestion tube. This helps the animal ingest only the food within the water when needed.
4. Tail
Unlike non-chordates, chordates typically have tail that run past the anus of the animal. The tail consists of muscle to help the animal move around, especially for animals that live in water.
5. Segments
Chordates are essentially metameric, or segmented, animals. This is shown in the figure above through the muscle segments of its body (8 BL). This repeating pattern can be seen clearly as the embryos develop as well. (2) (ER)


Acquiring and Digesting Food

Food particles enter the mouth along with seawater. The water passes through the pharyngeal slits and into the atrium, a small tube with an opening near the anus. Water moves out of the atrium and back into the sea. The food is trapped by a layer of mucous on the pharyngeal slits so none leaves through the atrium. The food then travels through the stomach where it is digested, and leaves by going out the anus.


Sensing the Environment

The class Cephalocardia use small eyes which lay in the exoskeleton to sense their environment. Their eyes are different than most crustaceans whose eyes are higher up on stalks. (HS 3) Oral cirri also act as sensory devices (and they filter water passing through the body) (SES 13). These are thin tentacle strands located in front of the mouth (SES 13).


Locomotion

Urochordates (tunicates) and cephalochordates (lancelets) have different ways of movement. Tunicates have different systems of propulsion in their life stages. Tunicate larva tends to look a lot like lancelets (see picture at the top of the page). They have muscles segmented along the length of their body and through the tail which moves side to side to push them through the water. Adult tunicates have a different way of moving through the water. They squirt water through the excurrent siphon which pushes them through the water. This odd way of movement has given them the commonly used term, sea squirt.
Lancelets, like tunicate larvae, have segmented muscles down the length of their body. The muscles move the tail side to side to allow the lancelet swim through the water.


Respiration

Lancelets take in oxygen by gas exchange. There are several areas on the outside of the animal that do this.
Lancelets perform gas exchange with the help of gills. In this process, they water is drawn through the mouth and expelled through the atriopore, an opening to the environment. While the water is still within the organism, it passes over the gill bars and gas exchange occurs by diffusion. (3) [KN]

Metabolic Waste Removal

For all invertebrate chordates, waste moves out the anus of the animal. Sea squirts have two openings in their body. The openings are called siphons; one functions as the mouth and the other functions as an anus. These siphons are arranged in such a manner that removed waste from the anus will not enter the siphone that functions as the mouth. Waste is propelled out of the excretory siphone using a current of water created by the organism (6, NK).


Circulation

Invertabrate chordates have a closed circulation system with blood vessels and hearts on their ventral side (belly side). [2] Sarah Vlach
They have a main ventral (bottom side) and paired dorsal (upperside) aorta (arteries bringing blood from heart to the rest of the bosy) that are connected by smaller vessels.They don't have one main heart but rather multiple hearts located in differentbranches of the circulation system. Because invertabrate chordates lack hemoglobin their blood is colorless (7). (RK)

Self Protection

Tunicates (urochordates) squirt water out their excurrent siphon, which includes the atriopore and anus, to propel them away from anything that is disturbing it.

seasquirt_46310.jpg
Here is a simplified diagram of a Tunicate illustrating
the movement of water in their self-defense mechanism (11 DJ)
Lancelets bury themselves in the sand while feeding. This behavior disguises them from predators. (14)(RJS)


Osmotic Balance


Osmotic balance is the control of keeping a regulated amount of water pressure inside the body. Lancelets do this by letting the water enter through the mouth. The water moves through the gills and then out through the atrium. The atrium is a small tube that leads water and other filtered materials that went through the gills out of the body. The atrium is different from an anus and is kept separate from the anus. Tunicates let water enter through their incurrent siphon, an opening on the top of their body that includes the mouth. The water passes through the gills and out through the outcurrent siphon, another openinig that includes the anus and the atrium.


Temperature Balance


Invertebrate chordates do not control their body temperature. It changes along with the animal's environment. There is a way that invertebrate chordates can control their body tempeature which is though their behavoir. Meaning when it gets to cold they move to a warmer place, perhaps out in the sun. Or when it gets too warm the move underground or into the shade.(SJB)

Reproduction
[HZ]
All tunicates are hermaphroditic (meaning they have both female and male reproductive parts). They often fertilize their own eggs. The gametes are released develop into tadpole-like larvae. The larvae swim to find a suitable place to settle down. Afterwards, they undergo metamorphosis to become tunicates (4). The lancelets, on the other hand, are either male or female. The eggs and sperm are released into the water, where they fuse to develop into larvae. When the larva has developed twelve to fifteen pairs of gills, it burrows into the sand and becomes a lancelet (5).


Review Questions
1.List and describe the 5 characteristics of this group. (AR)
2. What are the functions of siphons? (OZ)
3. How do tunicates move through water? And, how does the movement of 'immature' tunicates differ from that of 'adult' tunicates? {Jesse Carmen}
4. How does tunicate reproduction differ from lancelet reprodutcion? (VM)
5. How do lancelets and tunicates maintain osmotic balance? (DP)
6. What evidence is there that makes it is necessary to classify the two different types of invertebrate chordates? (SS)

Sources

Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.



A tunicate, also known as a sea squirt. Notice it's squirting motion. Fascinating.


Edited by: Sarah Vlach
Hilary Stepansky
Ethan Richman
Hanna Zhu
NK
Rachel Kornetsky
Becca Levenson
Kevin NayerJesse Carmen
Samuel Blatchford(SJB)
DP
SS
GR
Sarah Schwarzschild
Brittany Marcus-Blank
Meru Nangia

[2]http://faculty.fmcc.edu/mcdarby/Animals&PlantsBook/Animals/09-Basic-Chordates.htm#closed%20circulation
(3) Myers, P. 2001. "Cephalocarida" (On-line), Animal Diversity Web. Accessed December 02, 2008 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Cephalocarida.html.
(4) Shimek, Ronald L. "Tunicates of Sea Squirts: A Wet Link." Reefkeeping. 7 Dec. 2008 <http://www.reefkeeping.com/issues/2005-03/rs/index.php>.
(5) "Lancelets: Cephalchordata - Behavior and Reproduction." 7 Dec 2008 <http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html>.
(6)http://www.arctic.uoguelph.ca/cpl/organisms/inverts/marine_inverts/urochordates.htm
(7)Ramel, Gordon. "The Cephalochordata." Earth-Life Web Productions. 29 Sept. 2008. 7 Dec. 2008 <http://www.earthlife.net/inverts/cephalochordata.html>.
(8) "Phylum Chordata." Kingdom Survey-Animals. 2008. 7 Dec. 2008. <http://www4.alief.isd.tenet.edu/cahowe/AP%20Biology/powerpoints/PAK%2014%20Animals%2007.htm#slide0010.htm>
(9) "Biology 203 Lab." Biology 203 Lab. University of New Mexico. <http://biology.unm.edu/ccouncil/biology_203/summaries/deuterostomes.htm>.
(10) Cornell University. <http://www.biog1105-1106.org/labs/deuts/media/urochadult.jpg>.
(11)"Sea Squirt Image." Woods Hole Oceanographic Institution. 17 Dec. 2008.
(12) "Introduction to the Urochordate." UCMP Berkeley. 18 Dec. 2008 <http://www.ucmp.berkeley.edu/chordata/urochordata.html>.
(13) "Physical Features." Cephalochordate. 18 Dec. 2008 <http://www.nationmaster.com/encyclopedia/Cephalochordate>
(14)"Hints of primitive antibodies ." 18 Dec 2008 <http://findarticles.com/p/articles/mi_m1200/is_19_162/ai_94510935>.
15) "Characteristics of Chordates." 18 Dec. 2008 <http://www.shsu.edu/~bio_mlt/chap2.html>.
(16) "Urochordates." Bellarmine University Department of Biology. 19 Dec. 2008 <http://cas.bellarmine.edu/tietjen/images/urochordates.htm>.