Phylum Classification; Nematoda
Nematodes, commonly known as roundworms, are triploblastic psuedocoelomates. Most of the 15,000 discovered species are free-living and possess a common body plan. A thick outer covering composed of collagen-like proteins, called the cuticle, surrounds the bilaterally symmetrical body. The cuticle is permeable to gases and water to allow for osmoregulation for respiration and excretion, since Nematoda do not posses specialized organisms for these two processes. These organisms use an open circulatory system which pumps blood from the heart into a hemocoel in the body cavity causing blood to diffuse back to the circulatory system between cells.
The lack of a respiratory and circulatory system leaves an abundance of space within the body cavity. The body cavity contains a complete digestive tract for extra cellular digestion. A muscular gut traverses from the anterior end of the organism starting at the mouth to a subterminal anus. A nervous system rests alongside the digestive tract with longitudinal nerves that directly contact to muscle cells. The majority of the body cavity’s volume primarily hosts the reproductive organs, which can be tightly coiled in an individual. Most species are dioecious, meaning an individual will possess only a set of testes or ovaries, not both. Copulation between two individuals result in internal fertilization that will produce an offspring that has undergone chromatin diminution. Chromatin diminution’s function is not yet known, but results in the degeneration of a portion of chromatin that was eliminated during cell division. This elimination results in offspring which is genetically different from the parents reproductive cells.
The resulting offspring does not transition through a larvae stage, and instead presents as a smaller version of an adult Nematoda. The small “adult like” organism is born with the exact number of cells it will have for the remainder of its life. An organism that maintains the same numbers of cells throughout its life time is known as Eutelic. Over time, the cells will increase in size, rather than number, leading to an increase in the size of an organism. As an individual increases in size the organism will molt its outer cuticle in a process known as ecdysis.
The cuticle may contain amphids and phasmids which are the main distinguishing feature between the two classes of Nematoda, Adenophorea and Secrenenetea. The amphids are pouch-like invaginations of the cuticle that open to the exterior of the organism located near the mouth. The amphids contain ciliated receptors that may be used for sensory or excretory purposes. Phasmids are located at the tail region of some organisms and resemble the amphids, but are smaller and are used for chemosensory, secretory, or excretory purposes.
Class Adenophorea
Most species classified as Adenophorea are free-living organisms that inhabit marine, freshwater, and terrestrial ecosystems. Although most Adenophorea are free-living, some may have parasitic life-cycles. The paramount distinguishing feature of Adenophorea is that they presences of amphids and lack of phasmids. The amphids are located behind the lips on the anterior end of the organisms which function as chemoreceptors. The excretory system is another feature that differentiates Adenophorea species from Secernentea. The excretory system is comparatively simple and does not contain collecting tubules, excretory glands, or excretory canals.
Species Identification
Trichinella spiralis
Slides: In these images, the Nematode parasite Trichinella spiralis is observed in animal tissue. Under 4x magnification the small radial cysts are difficult to distinguish from the tissue, but as the magnification the details of the larvae become more apparent. At 40x magnification the encysted larvae are easily detected as one can observe circular forms within the tissue.
Class Secernentea
Most species of Secernentea are parasitic and are known to cause devastating diseases in humans, such as elephantiasis. Free-living species of Secernentea do exist as well and predominantly inhabit terrestrial ecosystems. Secernentea posses both cephalic amphids and caudal phasmids. The pore-like amphids are located on the lateral lips of the anterior end, while the phasmids are present on the posterior end and secrete an adhesive substance used to attach the organism to a substrate. Comparatively, the excretory system of these organisms is more complex than Adenophorea species. The system contains well developed collecting tubules, excretory glands, and excretory canals
Species Identification
Ascaris lumbricoides
Dissection Image:Female (top) and male (bottom) individuals are dissected to observe the internal structure of this species. Females are distinguished by their larger size, as males are smaller and have a curved posterior end.
Slides: The reproductive organs of Ascaris lumbricoides are observed in cross section. The male cross section show 4 round testes surrounding the central intestines. The female cross section displays two uteri filled with eggs northwest of the intestines, and circular ovaries scattered in the entire cross section.
Dissection Slides: During dissection images were taken of the curved posterior of the male individual to show the identification characteristic. Images were also captured of the uterus and intestine of the female individual.
Turbatrix aceti
Video: Turbatrix aceti,commonly known as Vinegar Eels, display movement patterns captured using a light microscope under 10x magnification. The whip-like motion is due to muscles that run along the sides of the organisms moving back and forth.
Nematoda Nervous Systems & Life Cycle
Phylum Classification; Rotifera
Rotifera species are triploblastic psuedocoelmates. These microscopic organism are most commonly found in fresh water, mosses, and terrestrial soils. They inhabit the thin layer of water the surrounds the particulate matter of these substrates. Rotifers usually range from 0.1 – 0.5 milimeters (mm) long, but some species can reach up to 2 – 3 mm.
The bodies of Rotifers are bilaterally symmetrical possessing a anterior and posterior end. The anterior end consists of a corona, a crown-like structure, of cilia forming a pre-oral band used for filter feeding. The mastax, or jaw, located inside the mouth aids in digestion. Food particles enter the complete digestive tract and exit through an anus on the posterior of the organism. The corona’s cilia are also used for movement when the organism is not attached to a substrate by its posterior foot. The unsegmented trunk of Rotifers contains most of the body cavity. Since these organisms do not possess circulatory or respiratory organs the cavity consists of digestive and reproductive organs. Excretion across the body surface replaces the respiratory system.
Rotifers are dioecious and undergo mictic and amictic life cycles depending on the species.In some species, only female individuals are observed that reproduce by a process called parthenogeneis. In this process the females asexually produce daughters from unfertilized eggs. In other species, females produce two types of eggs that when unfertilized can produce diploid females or haploid males. If a haploid male is produces cross breeding may occur if the male fertilizes another haploid egg. This egg will then develop into a female individual with altered genetics. Environmental conditions alter which types of reproduction will occur. In winter conditions, the environmental stimuli will initiate meiosis causing production of haploid eggs. Sexual reproduction aids maximizing the probability offspring will survive in unstable conditions due to in genetic variation being introduced through gene combination. Individuals may also be effected by environmental ques undergoing cyclomorphosis. This process produces different phenotypes from the same genotype. Genes will either be turned on or off depending on the stimuli.
Rotifers are classified into three classes; Monogonata, Bdelloidea, and Seisonidea. The classification depends on a species physiologic structure, environmental preference, and reproductive process.
Class Monogonata
Monogonata is the largest class of Rotifera containing approximately 1600 species. These species appear mostly in freshwater habitats as either sessile or free-living individual. The term monogonata refers to the unpaired ovaries in an individual. These organisms are distinguished by this single unpaired ovary. Monogonata species are dominated by female individuals throughout the year. These females exhibit mictic reproduction, and will produce diploid eggs that will develop, without fertilization, into daughters. Seasonally, when conditions become unstable, individuals will cycle reproduction methods to amictic reproduction producing haploid eggs. These haploid eggs if left unfertilized will produce males that may fertilize another individual haploid eggs to introduce genetic variation.
Class Bdelloidea
Bdelloidea have some of the most distinguishing characteristics of the Rotifera classes. The anterior end of these species contain a two-wheeled corona which surround the mouth of these organisms. The soft body is not enclosed with a shell-like outer surface, called a lorica. The movement of some species can be creeping as they move using cement glands similar to the caudal glands found on the posterior of Nematoda. The species in this class are completely amictic as no known males of these species have been discovered.
Philodina
Video: Two individuals of Philodina are observed under 10x magnification using a light microscope. One individual uses the spurs, or toes, on its foot to attach to a substrate. A different individual is observed free-swimming using cilia on its anterior end that would otherwise be used for filter feeding.
Class Seisonidea
Seisonidea is the smallest class of Rotifera consisting of only two marine Rotifers species. These species live on the gills of crustaceans, but are relatively large compared to the other classes growing to a few mm in length. The corona of the Seisonidea is severely reduces, and in turn a prominent mastax is observed. Unlike Bdelloidea and Monogonata, the ratio of male to female organisms in this class are relatively equal. Another distinguishing characteristic of these organisms is the appearance of paired ovaries not observed in the other two classes.
Rotifera Digestive System
Glossary of Terms
[1] Brusca, R.C., & Brusca, G.J.(2003). Invertebrates. Sunderland, MA: Sinauer Associates, Inc.
[2] Ruppert, E., Fox, R., & Barnes, R. (2004). Invertebrate zoology : A functional evolutionary approach (7th ed. ed.). Belmont, CA: Thomson-Brooks/Cole.
[3] Anderson, D. (2001). Invertebrate zoology. Melbourne: Oxford University Press.
[4] Meglitsch, P., & Schram, F. (1991). Invertebrate zoology (3rd ed. ed.). New York: Oxford University Press.
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