16.+chordata-+cephalochordate

Chordata- Cephalochordate Species: **Branchiostoma lanceolatum** By Payton H.




 * 1. Classification/Diagnostic characteristics**
 * Kingdom: Animalia
 * Phylum: Chordata
 * generally grow no larger than five centimeters
 * cephalochordates are also referred to as lancelets
 * 30 different living species of cephalochordates (6)


 * eel or snake like body shape
 * filter feeders
 * marine invertebrates (11)


 * earliest of chordatas to evolve
 * notochord prevents body from shortening
 * muscle fibers pulling together form a bend in the animal (9)

Cephalochordates are vertebrates, same as humans. These organisms also inhale oxygen and exhale carbon dioxide, despite their residency in water. Furthermore, cephalochordates have separate openings for their mouth and anus. (6) They are used as model organisms for studying evolution and development of vertebrae. (12) In China, lancets are sometimes consumed and they even support a small fishing industry. (7)
 * 2. Relationship to humans**

Cephalochordates are an aquatic species found in shallow marine and brackish waters. (6) They spend most of their time berried in sand, in shallow tropical and temperate oceans. (15) Mostly found in the southern North Sea and Kattegat, the Celtic Sea and Bay of Biscay. (1) The lancelet inhabits the sandy sublittoral zone at depths down to 30 m. (1) They prefer sand mixed with shells rather than muddy bottoms as it is not suited to penetrate ground with small particles. (1) In some places the they can have very large populations. Their population densities can be as high as several thousand individuals per square meter. (16)
 * 3. Habitat and niche**

The larvae of the European lancelet uses quick undulating movement and the ability to reverse its direction of swimming to avoid predators and to confuse predators as to which is the head end or tail end of its body. Once the European lancelet lives on the bottom of the seafloor, will hide most of its body in the sand by burrowing quickly when its senses a predator. (14)
 * 4. Predator avoidance**

The European lancelet has a mouth beneath its body to collect copepods, phytoplankton, and other suspended material during its free-floating descend. (2) First water is taking in through the mouth, drawn in by the beating of cilia on the wheel organ. The wheel organ is a set of ridges lying inside the mouth. Then the water is first filtered by the oral cirri and then it passes through the gill slits. Food particles are trapped by the mucus. The digestive contains a pouch or hepatic caecum which secretes digestive enzymes. (3) They collect their food using the pharyngeal slits or gill slits. (15)
 * 5. Nutrient acquisition**


 * 6. Reproduction and life cycle**
 * Gonads are formed in the second year
 * Adults mate in lancelets on the sea floor of specific deposits
 * Larve are fertelized externally, and develop in deposits
 * Adults live a maximum of 5-6 years (5)


 * 7. Growth and development**
 * all three clades of cephalochordates share the following developmental structures (6)
 * a hollow nerve cord in the organism's tail
 * a tail that protrudes behind the anus
 * a 'notochord' that extends through the whole body and provides support (6)
 * the notochord is composed of large cells with turgid fluid filled vacuoles, providing rigid yet flexible support (6)
 * Pharynx (throat)
 * develops around the pharyngeal slits
 * site of oxygen uptake and elimination of carbon dioxide and water
 * filters prey from water that passes through the gills and pharynx (6)
 * Brain and senses are very poorly developed- they do not have a true vertebrae (15)

Image- (18)

The body is segmented and flattened dorsoventrally with a membranous flipper along the dorsal side. The tail end has a lance-like flipper. (1) The skin has two regions, the epidermis and dermis. The epidermis is a single layered, thin layer of skin, and the dermis is connective tissue, and is much thicker than the epidermis. (17)
 * 8. Integument**

Cephalochordates can move both forward and backward through the gravel that they live in. They don't move around much outside of finding a good place to live, eat, and when they need escape predators. However, they move rapidly and burrow into the gravel and sand to escape predators, as seen below. (10) Lancelet is burrowing into sand through quick, wiggling movements. (10)
 * 9. Movement**

-sense organs are in the front end of the body (9) -there can be a brain due to an accumulation of nerve cells (9) -they have an extremely simple nervous system, with a single main nerve cord (20)
 * 10. Sensing the environment**

they have a gill, the vertebrate gill, from which they have gas exchange. (10) They have gill bars, which are supported by a branchial skeleton that is homologous to the gill skeleton of vertebrates. These gills are responsible for the filter feeding, as well as the gas exchange that takes place. The gas exchange is met by the thin epidermis of the general body surface. (13)
 * 11. Gas exchange**

Using a closed circulatory system that lacks hemoglobin, //B. lanceolatum// filtrates waste products across its own blood vessel into the water of the atrium that flushes out of the atriopore, thereby bypassing the use of a kidney. The pharynx holds water for oxygen to diffuse into the gills and CO2 to flow into the passing water that leaves through the atriopore. Other indigestibles leave through the anus at the posterior of //B. lanceolatum//. Walls of the atrium also contain cells that assist with waste excretion if those cells are in contact with the main blood vessels. Even the gonads use the expulsion of gametes to also deposit amounts of uric acid outside of the body. The varying excretory mechanisms of //B. lanceolatum// perhaps recall its marine nature that can easily take advantage of passing fluids in and out of the body for waste deposition, explaining the many locations of waste removal in this species. (8)
 * 12. Waste removal**

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. There are also nephridiopores that excrete nutrients. (21)
 * 13. Environmental physiology (temperature, water, and salt regulation)**


 * 14. Internal circulation**
 * //B. lanceolatum// blood is colorless and contains muscular blood vessels
 * //B. lanceolatum// don't have hearts or a __pericardial cavity__, which is a space on the heart that contains fluids to help the heart move more freely
 * Inside a //B. lanceolatum//, the blood flows from the __posterior end__, or the bottom of the //B. lanceolatum//, to the __anterior end__, or the top, through the ventral vessel, sub-intestinal vein, and the posterior cardinal veins
 * The blood flows from the anterior end to the posterior end through the dorsal aortae and the anterior cardinal veins (4)

This organism has a gland called the endostyle which produces some of the same hormones as the mammalian thyroid gland, including thyroxine. (19)
 * 15. Chemical control (i.e. endocrine system)**

1. Briefly explain the circulatory system in //B. lanceolatum//. The lancelet does not have a heart and its blood flows from the back of the organism to the front through muscular blood vessels. 2. How does this organism sense its environment? //B. lanceolatum// has sensory organs located at the front of its body. They have a bundle of nerves that act as a brain and a simple, singular spinal cord. 3. Where does the lancelet live? This aquatic organism resides in the sandy bottoms of shallow marine and brackish waters. 4. Briefly describe the life cycle of a lancelet. Gonads are formed during the second year of a lancelet's life. Mature organisms mate and the larvae are fertilized. Larvae develop in deposits where they were conceived. The typical lancelet lives for five to six years. 5. Describe the integument of a //B. lanceolatum//. The lancelet body is segmented and flattened dorsoventrally with a membranous flipper along the dorsal side. The tail end has a lance-like flipper.
 * Review Questions:**

Hillis, D. M. (2015). Principles of life. Sunderland, MA: Sinauer Associates. (6) http://www.ucmp.berkeley.edu/chordata/cephalo.html (3) and (15) http://species-identification.org/species.php?species_group=zmns&id=589 (5) and (2) http://www.marlin.ac.uk/species/detail/85 (1) https://www.biolib.cz/en/image/id12014/ (4) and (11) [|https://www.britannica.com/animal/cephalochordate#toc49503] (7) and (10) and (19) [] (8)  [] (8) http://www.biology-online.org/bodict/index.php?title=European_lancelet&redirect=no (12) http://lanwebs.lander.edu/faculty/rsfox/invertebrates/branchiostoma.html (13) http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1973.tb01381.x/full (14) http://www.biozoomer.com/2011/01/amphioxus-external-characters.html (14) http://eol.org/pages/1585/overview (16) disableHighlighting=false&displayGroupName=Reference&currPage=&scanId=&query=&docIndex=&source=&prodId=SCIC&search_within_results=&p=SCIC&mode=view&catId=&u=mlin_s_sharonhs&limiter=&display-query=&displayGroups=&contentModules=&action=e&sortBy=&documentId=GALE%7CCV2642050054&windowstate=normal&activityType=BasicSearch&failOverType=&commentary= (9) https://www.shutterstock.com/image-vector/lancelet-branchiostoma-lanceolatum-anatomy-scheme-vector-117398845 (18) https://oli.cmu.edu/jcourse/workbook/activity/page?context=b880ef1c80020ca601a28b3e327246e6 (17) http://faculty.washington.edu/chudler/invert.html (20) https://diversityoflife2012.wikispaces.com/Lancelet (21)
 * Sources:**