3.+Nematode

Species: //Caenorhabditis elegans//
 * Nematode ** - Jyalu Wu

Caenorhabditis elegans are microscopic, worm-shaped animals that are about 1 mm in length.

Classification:
 * 1. Classification/Diagnostic characteristics **
 * Eukaryotes: cells have nuclei (10)
 * Opisthokonts: choanoflagellates, animals, and fungi (10)
 * Animals: heterotroph made up of many eukaryotic cells (10)
 * Eumetazoans: has a symmetrical body, a gut, a nervous system, organized tissue layers, and specialized cell junction types (10)
 * Bilaterians: has bilateral symmetry and three tissue types: endoderm, ectoderm, and mesoderm (10)
 * Protosomes: mouth usually forms from a blastopore in early development (10)
 * Ecdysozoans: periodically sheds its cuticle (10)

Characteristics:
 * Nematodes are about 1 mm in length, have cylindrical bodies, a thick __cuticle__, which is a protective covering made of many layers of cells, and a body that isn't made of different segments. (10) (16) (7)
 * They can either be males or hermaphrodites, which are females that can produce their own sperm. (16)
 * They are microscopic animals with worm-like bodies and __stylets__, which are small and pointy appendages, that steal nutrients from plant cells. (7)


 * 2. Relationship to humans **
 * Nematodes are __model organisms__. (10)
 * Some nematodes are __parasites__. These nematodes include species such as the whipworm, which negatively affects humans and are deemed "pests." These species generally live in the intestines of humans and other animals and feed off of the human (or animal). In addition to their parasitic relationship to humans, they can potentially harm plans and lead to soil degradation. (15)
 * The __CED-3 protein__, which is involved in cell death, is similar to the gene in humans that controls cell death. This shows common ancestry since a similar gene is present in both species. (6)
 * They can also be planted in soil to remove infection.
 * Some species of nematodes can help humans by feeding on bacteria, fungi, protozoans and other nematodes, and playing an important role in releasing and cycling nutrients for plants. Additionally, some species attack insects and some have been commercially released as a biological insecticide. (9)


 * 3. Habitat and niche **
 * Most nematodes are parasites in other animals - others live in or on the ground. (10)
 * C. elegans are commonly found decomposing fruit. (3)
 * By consuming food, they excrete nutrients and minerals that are essential to plants.
 * Without them, plants nor animals would be able to get their needed energy from the soil.


 * 4. Predator avoidance **
 * C. elegans depend on their __chemosensory system__, which are organs that respond to chemical stimuli, to detect and feed on bacteria in its natural environment. (3)  (7)
 * 1) Their ability to detect repellent chemical cues produced by microorganisms, which triggers avoidance on the first encounter. (3)
 * 2) The second mechanism has to do with the ingestion of harmful bacteria by the nematode, which helps avoidance on future encounters. (3)


 * 5. Nutrient acquisition **
 * Nematodes exchange nutrients with the environment through the cuticle and the gut. (10)
 * Most nematodes are parasites and get their nutrients from their hosts. (10)
 * Many nematodes feed on protists and small animals. (10)
 * Nematodes pass nutrients through their gut by contracting an organ called the __pharynx__. (10)


 * 6. Reproduction and life cycle **
 * The reproductive system takes up most of the space inside the body. (2)
 * Hermaphrodites self-fertilize and can also mate with males. (2)
 * The single-celled worm is conceived after the sperm from either the hermaphrodite or the male joins with the egg. Then, __ embryonic cleavage __ ( the division of the embryonic cells) occurs, followed by the development of __ morphological __, or structural, characteristics. (2)  (7)
 * Nematodes have a 30-day life-cycle but can remain dormant for over 30 years.
 * They have a generation time of roughly 4 days.


 * 7. Growth and development **
 * Nematodes shed their cuticles four times during growth and development. (10)
 * The __zygotes__, or fertilized eggs, of nematodes can undergo __cytoplasmic segregation__, which is when the daughter cells of a cell with uneven distribution of some component in the cytoplasm will get different amounts of the component. (10)  (7)


 * 8. Integument **
 * Nematodes have distinct and well-organized tissue layers and have a thick cuticle made of many layers of cells. (10)
 * The skin of a C. elegans is made of a simple __epidermal__, or outer, __epithelial layer__, which is composed of a specific type of cells called epithelial cells, with a cuticle on top. (5) (7)
 * This skin layer covers the nematode and is simple and accessible. (5)
 * The epidermal cells form on the back surface of the C. elegans embryo and then differentiate to then enclose internal tissue. (5)
 * Nematodes also shed their skin throughout their lifetime.


 * 9. Movement **
 * //C. elegans// has 81 muscle cells arranged in 4 bands running from its head to its tail. (16)
 * //C. elegans// moves by flexing the length of its body in alternate directions, creating a wave-like curve to its motion much like a snake. However, these contractions are restricted to one direction from head to tail, requiring //C. elegans// to perpetually lie on its side. Unless in agar film, //C. elegans// also needs a certain depth of fluid to move efficiently: too low and the worms risk __desiccation__, or dehydration, and too high (deeper than the diameter of their body) and they cannot efficiently grasp the substrate to move across. Although able to move forward and backwards similarly, the worms exhibit increased contractions in the head, which allows easier reorientation. (8)


 * 10. Sensing the environment **
 * Despite lacking eyes and living in a dark environment, nematodes do have some sense of vision through light-sensing neurons, which they use to avoid light. (12)
 * Nematodes have brains that are circular and located around their mouth area; this brain is really just a ring of nerves that accounts for a lot of how they react to the environment. (11) (14)
 * Most of their responses to environmental stimuli come in the form of body __locomotion__, including thrash behavior. (11) (14)
 * Two main nerves: the __dorsal__ (back) nerve and the __ventral__ (belly) nerve, and the muscle cells in the nematode branch //toward// the two nerves instead of the other way around. (14)
 * Nematodes have a few tiny sense organs. (11) (14)


 * 11. Gas exchange **
 * Nematodes exchange oxygen with their environments through their cuticles. (10)
 * Nematodes, also known as __roundworms__, do not breathe like vertebrates but obtain oxygen through diffusion. (13)

Nematodes get rid of wastes through their anuses. They have quite an interesting digestive system that is composed of three main parts: the __stomodeum__, intestine, and __proctodeum__. These three parts resemble those of humans because it is a complete digestive track (has an opening, digestive tract, and exit). (10) A nematode is composed of many different parts, all with different functions. The intestine is part of its digestive system. @http://images.slideplayer.com/16/5262834/slides/slide_13.jpg
 * 12. Waste removal **


 * 13. Environmental physiology (temperature, water, and salt regulation) **
 * The habitats inhabited by nematodes are subjected to significant variations of temperature which has been shown to impact nematodes by affecting their ability to reproduce and thus their total population, including by influencing the survival rate of eggs and larvae, the time for gestation, and the gender of the offspring. (1)


 * 14. Internal circulation **
 * C. elegans do not have circulatory systems. (17)


 * 15. Chemical control (i.e. endocrine system) **
 * //C. elegans// have endocrine systems and are able to respond to stress and other changes in their environments by using it. They produce hormones, among other things. (4)


 * 16. Review Questions **
 * 1) What is special about the different sexes for nematodes?
 * 2) If C. elegans act and look very differently than humans, why are they used as model organisms?
 * 3) How can they be disadvantageous to humans?
 * 4) Why would nematodes shed their cuticles four times during growth and development?
 * 5) How does C. elegans move?


 * 17. Review Answers **
 * 1) Nematodes can be either males or hermaphrodites, which are females that can produce sperm.
 * 2) Like humans, nematodes are eukaryotes, require oxygen to survive, and have an outer layer of skin, a nervous system, and a digestive system. They share many of the basic characteristics of humans.
 * 3) Some species of nematodes are parasites and can infect humans or damage crops or the soil.
 * 4) The cuticle is the nematode's outer protective layer of cells, so it makes sense that it will be damaged and replaced during its growth period. Also, the nematode may shed the cuticle in order to make room for a larger one.
 * 5) By "swishing" its tail back and forth, nematodes are able to squirm around.


 * Sources **


 * 1) Anderson, Richard V, and David C Coleman. “Nematode temperature responses: a niche dimension in populations of bacterial-Feeding nematodes.” //PubMed.gov,// National Center for Biotechnology Information, www.ncbi.nlm.nih.gov/pubmed/19295677.
 * 2) Bahrami, Adam K, and Yun Zhang. “When Females Produce Sperm: Genetics of C. elegans Hermaphrodite Reproductive Choice.” //PubMed Central,// National Center for Biotechnology Information, 1 Oct. 2013, www.ncbi.nlm.nih.gov/pmc/articles/PMC3789810/.
 * 3) Ballestriero, Francesco, et al. “Caenorhabditis elegans employs innate and learned aversion in response to bacterial toxic metabolites tambjamine and violacein.” //PubMed Central,// National Center for Biotechnology Information, 7 July 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC4935850/.
 * 4) Baumeister, Ralf, et al. “Endocrine signaling in Caenorhabditis elegans controls stress response and longevity.” Journal of Endocrinology, 2006.
 * 5) Chisholm, Andrew D, and Tiffany I Hsiao. “The C. elegans epidermis as a model skin. I: development, patterning, and growth.” //PubMed Central,// National Center for Biotechnology Information, 19 June 2012, www.ncbi.nlm.nih.gov/pmc/articles/PMC3607643/.
 * 6) Ellis, H M, et al. “Ced-3 - Protein CED-3 Caenorhabditis elegans.” //WikiGenes,// Branco Weiss Fellowship, Society in Science, www.wikigenes.org/e/gene/e/178272.html.
 * 7) “English Dictionary, Thesaurus, & grammar help | Oxford Dictionaries.” //Oxford Dictionaries | English,// Oxford Dictionaries, en.oxforddictionaries.com/.
 * 8) “General Description of C. elegans.” //The Mind of a Worm,// WormAtlas, www.wormatlas.org/ver1/MoW_built0.92/description.html.
 * 9) Hay, Frank S. “Nematodes - the good, the bad and the ugly.” American Phytopathological Society, www.apsnet.org/edcenter/K-12/NewsViews/Pages/Nematodes.aspx.
 * 10) Hillis, David M. et al. Principles of Life. Sinauer Associates Inc., 2012.
 * 11) “Introduction to the Nematoda.” University of California Museum of Paleontology,www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html.
 * 12) Mo. “Nematodes see without eyes.” //ScienceBlogs//, 8 July 2008, scienceblogs.com/neurophilosophy/2008/07/08/nematodes-see-without-eyes/.
 * 13) “Nematode Respiratory System." //Study.com,// study.com/academy/lesson/nematode-respiratory-system.html#lesson.
 * 14) Schafer, William. “Nematode nervous systems.” Current Biology Magazine, 24 Oct. 2016.
 * 15) “What are Nematodes?” //University of Nebraska-Lincoln: Nematology,// nematode.unl.edu/wormgen.htm.12
 * 16) “What is C. elegans?” College of Biological Sciences, University of Minnesota, cbs.umn.edu/cgc/what-c-elegans.
 * 17) <span style="font-family: Arial,Helvetica,sans-serif;">“What is C. elegans?” //Developmental Neurobiology Laboratory,// Simon Fraser University, Department of Biological Sciences, www.sfu.ca/biology/faculty/hutter/hutterlab/research/Celegans.html.