Chordata+–2+invertebrate+subphyla

**//[|5]Subphylum Urochordata and Subphylum Cephalochordata//**
Lee Johson

(NG)

The invertebrate subphyla of the chordates are the subphylum Urochordata, commonly know as **tunicates**, and the subphylum Cephalochordata, commonly know as **lancets**. These two subphyla are similar in the fact that they display a simplified, invertebrate body plan of more complex chordate species. Tunicates are "marine filter-feeding animals" [MS] [|5] that, however, only display the characteristic anatomy of chordates in the larva form, and loose their chordate like phenotypes when they undergo metamorphosis. For tunicates, "i f the larvae became sexually mature without developing tunicate characteristics, the urochordate larva may have been ancestral to vertebrates."(MLK)[|7]



__ Defining Characteristics __
The diagnostic characteristics that define the 2 invertebrate subphyla of the phylum Chordata include the four anatomical features that characterize the phylum Chordata which are: a) A **notochord**, which is a longitudinal, flexible rod located between the digestive tube and the nerve chord, which provides skeletal support for the animal. b) A hollow nerve chord, located dorsal to the notochord. c) **Pharyngeal slits**, which are located posterior to the mouth and allow water to pass through the animal without traveling through the digestive tract. This allows for affective suspension feeding. These Pharyngeal slits have evolved into structures such as gills, ears, and jaw support in advanced chordates. d) A muscular tail that extends posterior to the anus.

The invertebrate subphyla of chordata differ from the other chordata due to the absence of a backbone, the absence of a cranium (skull), the absence of neural crest cells, the absence of an elaborated brain, and the absence of paired sensory organs on the head. (13)(MF) media type="youtube" key="-Jooz4gz264?fs=1" height="385" width="640" Sea Salps, a kind of tunicate, are just one beautiful creature belonging to this subphyla. (JS) (17) Both lancets and tunicates feed through **suspension feeding**. Tunicates pump water into their mouth by an **incurrent siphon**. The water passes through a mucous net located on the **pharynx.** Food particles are filtered from the water and passes into the intestines. The filtered water passes through the pharyngeal slits into the **atrium**, and exits through an **atripore**, or **excurrent siphon**. Similarly the lancet secretes a mucus membrane across its pharyngeal slits. This membrane traps food particle from water that is pumped through the mouth by **ciliary pumping**. The water exits through the pharyngeal slits and through an atriopore.
 * __ Acquiring and Digesting Food __**

In cephalochordates, enzymes are secreted by a special pouch known as the hepatic caecum, and actual digestion of food occurs in a particular part of the intestine called the iliocolonic ring. (NI) [|2]

"A groove in the pharynx called the endostyle secretes mucus that traps the particles and conveys them into the digestive tract; the movement of the mucus is caused by the action of cilia. Water leaves the atrium, a sac surrounding the pharynx, by way of the excurrent siphon. Thus the gill slits in tunicates serve a feeding function, not a respiratory function." [MS] [|5]

 media type="youtube" key="GUR3sQR9nUY?fs=1" height="385" width="640" A tunicate filter feeding. (MC)

**__ Metabolic Waste Removal __**
In the tunicate, waste is excreted from the anus into the atrium, where it is removed from the body w ith the filtered wather from the pharynx throught the excurrent siphon. The lancet however excretes waste through the anus directly into the surrounding, not into the atrium.



__ Respiration __
Interestingly the pharyngeal slits, gill like structures, only have a minor affect on a respiration, and most gas exchange takes place across the exterior body surfaces of the lancet. Essentially, lancets have no respiratory system. Gas exchange takes place through their skin, which is composed of simple epithelium, or one layer of cells. The gill slits are mostly useful for feeding. (DB) (2) 

__** Circulatory System **__
The tunicate pumps water in through its incurrent siphon, through its pharyngeal slit, and out through its excurrent siphon. This acts as its circulatory system. [Cephalochordates have a well-developed circulatory system that is composed of paired **nephridium.** Nephridium function liked kidneys, which also help in the excretory system. (MP)[|3]]

Tunicate blood is pumped around its body through numerous small spaces called **sinuses**. These sinuses perform the same functions as blood vessels. The tunicate heart beats 100 beats during which the blood flows in one direction. The heart then stops beating for a while. It then starts beating again, and the blood moves in the opposite direction. Tunicate blood is often clear. (LW) (12)

**__ Locomotion and Self Protection __**
The lancet and tunicate have very different life cycles. The tunicate begins its life as a free swimming (propulsion is provided by a muscularly segment tail), non-feeding larvae until it attaches by its head to a solid surface such as a rock, dock, or boat and stays. At this point it undergoes a metamorphosis to its adult form where it looses most of its chordate characteristics. During this metamorphosis, the tunicate loses its tail, ability to move, and most of its nervous system.(KL)([|16]) However other species of tunicates can be planktonic or colonial. Lancets burry themselves in the sand leaving their anterior end exposed. They frequently relocate and re-burry themselves in a new location. To do so lancets use a simple side-to-side swimming motion of their anterior. This motion is produced through the coordinated contraction of rows of muscles along the notochord, flexing the notochord.

The muscles for lancets are arranged in a v-shaped pattern and show evidence of segmentation. The muscles have a single nerve that branches from their main nerve cord. This is a similar but more primitive version of how our nerves branch off from our spinal cord: this observation has been part of the reason why it is believed that we have a common ancestor with invertebrates (SD) (5).  (CSR [|15])

**__ Self Protection and Sensing of the Enviroment __**
Tunicates protect themselves by shooting a jet of water through their excurrent siphon when they are molested. They use sensory receptors to process this molestation and react. "Cephalochordates have a simple dorsal nerve cord enclosed in a sheath of collagen fibers which runs the length of the body below the notochord (a cord of cells that supports body structure). From this arise a series of smaller nerves which serve the rest of the body. There is no brain or cranium." (AK) (4)

The small sensory like tentacles above surrounding the mouth, are seen in the lancelet organisms.(YA)(9)

__** Temperature Balance **__
As aquatic invertebrates, tunicates and lancets are **thermoconformers** with little control over thier body temperature.

Thermoconformer's body temperature, most of the time, corresponds to the overall environmental temperature. (AC)(8)

Although the invertebrates are thermoconformers there is a way for them to have some control over their body temperatures. The chordates can control their temperatures through behavior, for example when it gets too cold they may move into the sunlight. When their bodies get too hot they may move into the shade or underground.(CW)(6)

__** Osmotic Balance **__
Both lancets and tunicates are **osmoconformers,** meaning there osmolarity is the same as that of the surrounding sea water. They do however regulate their internal compostion somewhat, because the concentration of specic solutes in them is different than that of the surrounding water. Freshwater invertebrates are osmoregulators, and face the challenge of being hypertonic to their environments. They ingest large amounts of water to get the salts they need, and then prevent swelling by releasing large quantities of dilute urine. Marine invertebrates, however, are usually osmoconformers and have little trouble obtaining the salts they need from their environment. (ZXU)([|10])

Reproduction: Tunicates are hermaphroditic, which means that they possess both male and female reproductive systems. To avoid self-fertilization they generally release sperm and eggs that are not chemcially compatible, or release the sperm and eggs at slightly different times.The sperm is released into the water while the eggs stay in the body. The egg is fertilized when the sperm is brought in through the current of water. The larvae of tunicates resemble tadpoles, and have the ability to swim. After hatching, the find a spot on the sea floor in the "head down, tail up" position; they adhere to the floor through special glands on their head. Once they are attached to the floor, they undergo miraculous development and become tunicates. (RG) [|(14)]

1. Describe and explain the life cycle of the tunicate. How is it different than the lifecycle of the lancets? (CC)
 * Review Questions: **

2. Describe the innate behavior of invertebrates in terms of kinesis. (MT)

3. A food particle is eaten, then expelled as waste. Explain how a tunicate OR lancet would digest it, then explain how the other (the one not chosen for digestion) would excrete the waste. (TM)

4. Compare and contrast how lancets and tunicates acquire and digest food. (SM)

5. What are pharyngeal slits? How do they show the relation of tunicates and lancets to more advanced chordates? Why are they important?(ORS) 6. What distinct/specialized parts do tunicates have that allow them to sense their environment? (KS)8

7. What are the four major anatomical characteristics of subphylum Urochordata and subphylum Cephalochordata? (ZS)

8. How are Tunicates similar to some reptiles in their thermoregulation ability? explain. (GR)(17)

9. Explain how both lancets and tunicates regulate their internal composition as osmoconformers. (SI)

10. Describe what you have learned about the nervous system. (RL)

Sources: 1. Campbell, Neil A, and Jane B. Reece. . Sixth Edition. San Fransisco: Pearson Education, Inc, 2002. 2. [] 3. http://www.ucmp.berkeley.edu/chordata/cephalo.html (Meaghan Perry) 4. [] 5.  __[]__. 6. [] 7. [] 8. http://www.annualreviews.org/doi/abs/10.1146/annurev.ph.57.030195.000441?journalCode=physiol 9. [] 10. http://www.cartage.org.lb/en/themes/sciences/zoology/animalphysiology/osmoregulation/osmoregulation.htm (ZXU) 11. [] (NG) 12. [] (LW) 13. http://cas.bellarmine.edu/tietjen/images/urochordata.gif (MF) 14. [] 15. http://pigeonchess.com/2008/06/22/you-can-tune-a-piano-but-you-cant-tunicate/ (CSR) 16. [] (KL) 17. [] (JS)