LanceletAnatomy.gif
An adult lancelet burrowed in sand. (7) (SJ)


1. Classification/ Diagnostic characteristics

Kingdom: Animalia
Phylum: Chordata
Subphylum: Cephalochordata
Class: Leptocardii
Order: Amphioxiformes
(8)(JLau)



Lancelets are Cephalochordates, which is a type of chordate that develops a hollow dorsal nerve cord, a tail that goes farther than the anus, and a notochord that is supporting rod located on the back of the organism. To go even further, chordates are a type of deutersomes, where the mouth is created at the other end of the embryo away from the blastopore. At the blastopore, the anus is formed Also deutersomes are bilaterally symmetrically and are segmented .They also have a pharynx with slits in them so that water can flow in and out.
The lancelet has a dorsal notochord, or stiffening rod, extending from tip to tail, that gives it its characteristic pointed shape. It retains the notochord as the major skeletal support throughout life. (TM)
A Lancelet
A Lancelet
(14)(KG)

There are around 30 lancelet species, many from the genus Brachiostoma (formerly Amphioxus). Lancelets usually are 1 inch (2.5 cm) long with transparent bodies that taper at both ends. There is no distinct head. They are small, fish like chordates, with many similarities to vertebrates [1] (SM)

2. Relationship to humans
Lancelets: which have notochords gave rise to vertebrates.

Humans eventually arose from lancelets due to two successive rounds of DNA doubling. Initially, the invertebrate organism accidentally doubled its DNA, and then later offspring made the replication mistake again. This series of events eventually led to the rising of various other animals and humans (10) (E.S.S.).

Because of these pivotal evolutionary events, lancelets are extensively studied by researchers, as they provide key information about how the phylum of vertebrates evolved. More importantly, in studying lancelets, scientists are trying to determine how the duplicated genes evolved new functions that led to higher evolution in vertebrates. Research in lancelets are providing many answers to the evolutionary puzzle of how humans came into being. [2] (FZ)

3. Habitat and niche
Lancelets live in the shallow parts of water and salty water.
In the warmer and temperature parts of the world, the lancelets spend the majority of their time buried in mud, sand, or gravel on the ocean floor even though they are capable of swimming. They do this especially while feeding so they can filter food particles from the water passing through their gill slits. (9)(WSS)

4. Predator avoidance
Lancelets burrow in clean gravel or sand with just the head exposed. When they are distrubed, lancelets leave their burrows, swim quickly for a short distance, and rapidly burrow again with the posterior end in the substrate. (5) (JF)
Nutrient acquisition
They clean their prey captured from the water with the pharyngeal basket slits.

5. Nutrient Acquisition
Lancelets feed on plankton, microscopic plants and animals drifting in water, as well as diatoms, a type of algae that has a shell. (6) (JF)

During feeding, lancelets use the beating of cilia inside the mouth to draw in water. Slender projections called oral cirri surround the opening of the mouth and first filter the entering water. It then passes through the gill slits, which are enclosed within the body wall in a body cavity known as the atrium. As the water passes through the slits and out of the atrium, food particles in the water are trapped by secreted mucus. From there, a pouch called the hepatic caecum secretes digestive enzymes, although the actual digestion takes place in the iliocolonic ring, a specialized part of the intestine. (4) (JLev)

Depending on the type of sand, lancelets feed differently. In coarse sand, their entire body is buried while just their head is exposed for nutrients to enter. In fine sand, lancelets lie on the bottom with their entire body exposed to the water. (LC) 116. Reproduction and Life CycleLancelets reproduce sexually where each gamete comes from each parent. The gamete is composed of a chromosome from the mother and father, which comes from a homologous chromosome pair. The gametes are haploid or n when the two haploid gametes join together it creates a zygote or 2n- this is know as fertilization. The zygote further divides to create a more complex organism, however this is not certain with lancelets since they are primitive organisms.
The left diagram shows very early cellular development of a lancelet and the left shows important structures on adults (16) (BH)
The left diagram shows very early cellular development of a lancelet and the left shows important structures on adults (16) (BH)
7. Growth and DevelopmentLancelets do not really grow beyond 5 centimeter (2.2 inches), but their notochords grow their entire lives. Since lancelet are technically chordates, they develop an empty dorsal nerve cord, a tail that goes farther than the anus and a notochord which is a supporting rod locate on the back of the organisam. Lancelets are devtersomes, which develop the characteristic of the mouth being created at the other end of the embryo away from the blastopore.
Spawning, or when the lancelet deposits eggs, occurs around of after sunset. The breeding season tends to be during the spring and summer. Eggs and sperm are released into the water out, so fertilization is external. Little yolk is contained in the egg and the embryo develop quickly into freely swimming larvae. The larvae then swim upward in the water column and remain there as plankton for a variable amount of weeks (duration dependent on temperature). The development from larvae to the juvenile stage is generally gradual. The lifespan of the lancelet is unusually long (2 to 5 years). (3) (PS)
Cephalochordates have separate sexes. Spawning occurs around or after sunset. Eggs and sperm are released into the water flowing out of the atrium and fertilization is external. The eggs contain little yolk and and embryos develop rapidly into free-swimming larvae. Larvae swim upward in the water column where they remain as plankton for weeks (the duration varying with temperature). When feeding, larvae swim upward, then hover or slowly sink passively with the body held horizontally and the mouth directed downward, feeding on plankton and other suspended material. Development from the larval to juvenile stage is generally gradual. (Brusca and Brusca 2003) Lancelet embryology is reviewed by Whittaker (1997). Lancelet life spans are surprisingly long, 2 to 5 years depending on the species, with a spring and summer breeding season (Stokes 1996, Whittaker 1997).8. IntegumentAt the blastopore, the anus is created.
On the lancelets skin certain cells are capable of producing a mucous to act as a boundary between the seawater and seafloor and the organism. There is evidence that the lancelets skin has a very primitive form of keratinization to protect itself. (15) (DA)

Keratinization starts when protein in the form of food is digested and the body breaks it down into amino acids. The Keratinization Process takes approximately 28 days. However, as you mature, the process slows down and takes longer to complete. These enter the blood stream and are carried to the papilla and the matrix at the base of the follicle where cells are formed that will eventually create hair.(HSC)

9. MovementWhen Lancelets are in their adult stage, they are sessile or they do not move. They bury themselves in the sand with only their heads sticking out of the sand. Although they are sessile,lancelets are able to swim.Although the adults do not move much the larvae may move drift great distances before settling in one spot. (MDS) [3] 10. Sensing the EnvironmentLancelets have sensory receptors that turn stimuli senses like touch, light or sound to action potentials or changes in ion concentrations in the cell membrane which leads to a electrical charge. The electrical charge goes through axons to finally give off a chemical signal. This goes to the central nervous system to be translated. There are five kinds of sensory receptors – Mechanoreceptors, Chemoreceptors, Thermo receptors, electro sensors and photoreceptors. Lancelets may have had mechanoreceptors to sense physical forces like touching or sound. It may have also had chemoreceptors, which controls the amount of chemicals in the body and electro sensors which sense changes in the ion concentrations in the cell membrane. Lancelets have thermo receptors, which senses changes in temperature and photoreceptors that senses the presence of light.11. Gas ExchangeLancelets probably do gas exchange through diffusion in water. Since obtaining oxygen from water takes a long time, oxygen take in by the organism is in small amounts. CO2 diffuses out of easily into water.
Lancelets, absorb some O2 through the skin, but some like other fish is absorbed through the gills. Gas exchange occurs as water passes over the gill bars. (18) (BS)12. Waste RemovalSince they are aquatic organism invertebrates, Lancelets probably excreted ammonia because ammonia can dissolve in water quickly. It loses ammonia due to diffusion of it across its skin.
After water is drawn in by the beating of cilia and filtered by small appendages, oral cirri, it will pass through the gill silts. The water is filtered out of atrium and food particles are sent to the iliocolonic ring, a specific region in the intestine where digestion occurs. The hepatic caecum, an endrocine gland, will secret digestive enzymes. Wastes are excreted by the anus. (AWC) [4]

13. Environmental Physiology
Lancelets have protonephridia, a network of small tubules without internal openings. These tubules are used to remove waste fluids and smaller solutes while retaining larger proteins within the body. At the end of these are nephridiopores, small pores on the outside through which the nutrients are removed. This method was originally developed to osmoregulate water content, but it has also adapted for other ions. (17) (DM)

Lancelets are sea creatures that live deep in the ocean, but most importantly, they are in fixed at their respective locations, such as a boat or rock. (ES)

14. Internal CirculationLancelets must have had gills to oxygenate the blood.
Lancelets have a closed circulatory system with no central heart. Instead, there are branchial hearts (muscular veins located in the gills which contract, pumping the blood)t hat supply the oxygenated blood to the main aorta (the artery that transports the blood to the rest of the body) which branches into tiny vessels that bring that exchange the oxygenated blood to the organs. Interestingly, Lancelets do not have hemoglobin, leaving their blood transparent.[2] (AY)
Lancelet Circulatory System (MC)
Lancelet Circulatory System (MC)


15. Chemical ControlIt is probable that lancelets had a very simple endocrine system. Endocrine system consists of cells that release substances into extra cellular fluid or inside a body part that goes to the outside of the body. Lancelets may have released hormones endocrine signals or cells that release substances into extracellular fluid, which come from endocrine glands. When hormone signal released, the target cells hormone known and bind to the hormone to get a response.
Review Questions:1. What are a lancelet's mechanoreceptors, chemoreceptors, thermoreceptors, electrosensors and photoreceptors? How do they contribute to the organism's ability to detect it's environment? (AA)
2. Describe the functions of protonephridia and nephridiopores in lancelet's environmental physiology. (JF)

References
  1. ^







    http://www.factmonster.com/encyclopedia/science/lancelet.html
  2. ^






    http://berkeley.edu/news/media/releases/2008/06/18_lancelet.shtml
  3. ^
    http://animals.jrank.org/pages/1666/Lancelets-Cephalochordata.html
  4. ^
    http://en.wikipedia.org/wiki/Lancelet

1. Hillis, David M., David Sadava, H. Craig Heller, and Mary V. Price. Principles of Life High School Edition. Sunderland, MA: Sinauer Associates, 2012. Print.2. http://www.earthlife.net/inverts/cephalochordata.html3. http://eol.org/pages/1585/overview4. http://www.ucmp.berkeley.edu/chordata/cephalo.html5. http://eol.org/pages/1585/overview6. http://animals.jrank.org/pages/1666/Lancelets-Cephalochordata.html7. http://biology.unm.edu/ccouncil/Biology_203/Summaries/Deuterostomes.htm8. "Lancelet." Wikipedia. Wikimedia Foundation, 12 Sept. 2012. Web. 13 Dec. 2012. <http://en.wikipedia.org/wiki/Lancelet>.9. http://www.britannica.com/EBchecked/topic/21580/amphioxus10. Viegas, Jennifer.
"500-Million-Year-Old 'Mistake' Led to Humans." Discovery News. N.p., 24 July 2012. Web. 13 Dec. 2012. <http://news.discovery.com/animals/dna-mistake-evolution-120724.html>.
11. http://animals.jrank.org/pages/1663/Lancelets-Cephalochordata-BEHAVIOR-REPRODUCTION.html


13. http://www.factmonster.com/encyclopedia/science/lancelet.html
14.Hillewaert, Hans. 1997. Photograph. Belgium.
15. http://link.springer.com/article/10.1007%2FBF00384200?LI=true
16. http://www.geochembio.com/IMG/lancelet-embryology-anatomy.png
17. http://www.biology.ualberta.ca/courses.hp/zool250/animations/Excretion.swf

18. http://biology.unm.edu/ccouncil/Biology_203/Summaries/Deuterostomes.htm