unit 3 a.docx

Architectural patterns in animals Amongst animals there are five major grades of organisation, with each one more complex than the preceding one. 1. Protoplasmic grade of organisation Found in all unicellular organisms. All life functions are confined within the boundaries of a single cell. Within t...

Architectural patterns in animals Amongst animals there are five major grades of organisation, with each one more complex than the preceding one. 1. Protoplasmic grade of organisation Found in all unicellular organisms. All life functions are confined within the boundaries of a single cell. Within the cell, protoplasm is differentiated into organelles capable of performing specialised functions. 2. Cellular grade of organisation Cellular organisation is an aggregation of cells that are functionally differentiated. A division of labour is evident with some cells concerned with reproduction, others with nutrition. Such cells have little tendency to become organised into tissues (a group of cells performing a similar function). Animals in this group include flagellates such as volvox (phylum Chlorophyta) and some place sponges (phylum Porifera). 3. Cell tissue grade of organisation The next stage involves aggregations of cells into particular layers. Some researchers place phylum Porifera in this group, along with Cnidaria (jellyfish and relatives), which clearly show tissue organisation in their neural net. 4. Tissue-organ grade of organisation An aggregation of tissues into organs is the next step in complexity. Organs are usually comprised of more than one type of tissue. This is the organisational level of flatworms (Platyhelminthes). 5. Organ-system grade of organisation This is the level at which organs are integrated into systems to perform complex functions. Systems are associated into basic body functions such as circulation, respiration and digestion. The simplest animals with this degree of organisation are nemertode worms. Most animals display this level of complexity. Animal symmetry Symmetry refers to balanced proportions of parts on opposite sides of a median plane. Spherical symmetry (any axis through the centre of the animal is a mirror image of the other), e.g. some unicellular forms like Volvox. Radial symmetry applies to forms that can be divided into similar halves by more than two planes passing through the longitudinal access, e.g. some tube of vase shaped sponges, hydra, jellyfish and sea urchins. Bilateral symmetry applies to animals that can be divided into left and right halves (the majority of animals). Some terms, which are used to locate areas on bilaterally symmetrical animals, are: Anterior - the head end Posterior - the opposite or tail end Dorsal - the back (in most cases top) side Ventral - the front, or belly side Medial - midline of the body Lateral - sides of the body Distal - further from the middle of the body, e.g. foot of a vertebrate Proximal - nearer the central body, e.g. upper limb of a vertebrate Body cavities A major evolutionary step for bilaterally symmetrical animals is a fluid filled space surrounding the gut. Such a space (pseudocoelum or coelom) provides a tube within a tube arrangement that allows greater flexibility of the body cavity. The coelom also provides space for visceral organs and permits greater size and complexity by exposing more cells to surface exchange. The coelom also functions as a hydrostatic skeleton, especially in worm locomotion. Endoderm Acoelomate Mesoderm Ectoderm Psudocoelomate Gut Psudocoel Ceolom Coelomate Cephalisation Differentiation of a head end (cephalisation) is found in all bilaterally symmetrical animals. The concentration of nervous tissue and sense organs at the front of the animal is advantageous for animals engaged in linear locomotion. Usually the mouth is also located on the head, as much of the animal's activity is procuring food. Segmentation (metamerism) Metamerism is a serial repetition of similar body segments along the longitudinal axis of the body. Each segment is called a metamere or somite. In organisms such as earthworms, the segmental arrangement includes both internal and external structures of several systems, including muscle, blood vessels, nerves and the seatea of locomotion. Other organs such as reproductive organs may be repeated in only a few somites. Metamerism permits greater body mobility and complexity of structure and function, and is a major step forward in evolutionary history. Metamerism is found in phyla Annelida and Chordata but most significantly in phylum Arthropoda, the largest assemblage of animals on Earth. Taxonomy and phylogeny Zoologists have named over 1.5 million separate forms of animal life, with thousands more being described every year. All human cultures classify animals familiar to them according to different criteria, such as their usefulness/destructiveness to human activity, or according to mythology. Biologists group animals according to evolutionary relationships as revealed by homologous features. Taxonomists have three goals, to discover all species of animals, to reconstruct their evolutionary relationships and then to classify them accordingly. Linnaeus and the development of classification Carolus Linnaeus (1707 to 1778) arranged living organisms into an ascending series of groups of ever increasing inclusiveness, a hierarchical system of classification. The groups were given ranks to indicate the degree of inclusiveness of the group. There are 12 ranks shown below in the classification of human beings and a Katydid (grasshopper-like insect). Rank Human Katydid Kingdom Animalia Animalia Phylum Chordata Arthropoda Subphylum Vertebrata Uniramia Class Mammalia Insecta Subclass Eutheria Pterygota Order Primates Orthoptera Suborder Anthropoidea Ensifera Family Hominidae Tettigoniidae Subfamily \-\-\-\-\-\-\-\-\-\-\-\-- Phaneropterinae Genus Homo Scudderia Species Homo sapiens Scudderia furcata Subspecies \-\-\-\-\-\-\-\-\-\-\-- Scudderia furcata furcat All organisms must be placed into a least seven taxa, one of each of the mandatory ranks. Other subdivisions, in addition to the ones mentioned, can be superclass, infraclass, superorder, etc.) Linnaeus' system for naming species is known as binomial nomenclature. Each species has a Latinised name composed of two words, printed in italics (or underlined if handwritten). The first word is the name of the genus which is written with a capital initial letter, followed by the species epithet, which is unique to the species within the genus and is written all in lower case. The genus name is always a noun, and the species epithet an adjective, e.g. Turgus (Latin for thrush) migratorius (of migratory habit) is the scientific name for the common robin, a member of the thrush group. If subspecies exist within the species, the individual is referred to with a three word name, comprising genus, species and subspecies e.g. Ensatina escholtzi foregoneness, and Ensatina escholtzi platensis, both members of the same salamander species which differ in appearance and geographical distribution. Part 2: simple animals Unicellular organisms, protozoan groups Introduction Life on earth dates from 3.5 billion years ago, consisting of bacteria like prokaryotes. Unicellular eukaryotes are thought to have developed via the process of symbiogenesis. Certain aerobic bacteria would have been engulfed by other bacteria that were able to cope with the increasing amounts of oxygen in the early atmosphere. The aerobic bacteria had the enzymes necessary for producing energy in the presence of oxygen. These engulfed bacteria became the ancestors of mitochondria. Some ancestral unicellular eukaryotes engulfed photosynthetic bacteria, (which became chloroplasts, photosynthetic plant organelles) which eventually resulted in multicellular plants. Some unicellular eukaryotes that did not develop chloroplasts (and some that did) evolved animal-like characteristics, producing the variety of organisms that are collectively called Protozoa. They are distinctly animal-like in that they lack a cell wall, have at least one motile stage, and most ingest their food. Protozoa are found everywhere life exists. They are highly adaptable and can easily distribute from place to place. They require moisture, and live in marine or freshwater habitats, soil, decaying matter, and in living plants and animals. About 15% of the described species (over half of these are fossils) are symbiotic within or on other living organisms (animals, plants, other protists). The relationship may be mutuality (both partners benefit) communalistic (one partner benefits, no effect on the other) or parasitic (one partner benefits at the expense of the other), depending on the species involved. The group can be divided into four classes; flagellates, amoebas, sporozoans (an important parasitic group, including malarial organisms) and ciliates. Movement Protozoa move mainly by cilia and flagella and by pseudopodia movement. Structurally, cilia and flagella are similar, except cilia propel water parallel to the surface to which it is attached and flagella propel water parallel to the flagella itself. Ciliary movements are used by protists not only for locomotion but also to generate water currents for feeding and respiration. Pseudopodia are the chief means of locomotion in amoebas, and they can be formed by a range of flagellate protozoa, as well as by amoeboid cells of many invertebrates and vertebrates (e.g. white blood cells). The cytoplasm inside the cell is capable of changing states from a fluid (plasmasol) into a more solid state (plasmagel) and vice versa. When the organism locomotes, the plasmasol flows through the centre of the cell towards the direction of movement. When the plasmasol moves to the sides, it becomes solid again. This way the cell cannot only propel itself as a whole, but can also send pseudopodia in many directions. Nutrition Protozoans can be categorised as either autotrophs, which synthesise their own organic components from inorganic substrates or heterotrophs, which obtain organic molecules synthesised by other organisms. Heterotrophs, which ingest visible particles, are called phagotrophs. Those, which utilise soluble food, are termed osmotrophs. Confusingly, individual species can be both autotrophic and heterotrophic. Phagocytosis involves the invagination of the cell's membrane around a food particle, which is pinched off to form a food vacuole or phagsome. Lysosomes within the cells join with the phagsome releasing their digestive enzymes. Digestive products are absorbed into the cell and undigested remains are ejected from the cell in the same manner. The identical process involving liquid material is termed pinocytosis. Reproduction Due to the diverse nature of the group, many methods of reproduction occur. Fission is widespread, most commonly binary fission, although multiple fission also occurs in some groups such as Apicomplexa and some amoeba. Budding occurs in some ciliates. Protozoa also reproduce sexually, with gamete nuclei formed with individual cells. They can be exchanged with other cells (or fused within the same parent cell) before fission occurs. Many protozoa also survive periodic harsh environments by the ability to form cysts, a dormant form with a strong impervious outer coating. Parasitic forms make use of cysts when between hosts. Phyla Retortamonda (Giardia) and Axostylata (Trichomonas) Phylum Retortamonads (Giardia) and Axostylata (Trichomonas) Rather a small group in terms of numbers, but they are important to humans due to their disease-causing effects. Giardia is an intestinal parasite of humans and other animals causing symptoms of mild to moderate discomfort and diarrhoea. Cysts are passed in faeces and new hosts are usually affected by drinking infected water. Trichomonads can be important as some species (e.g. Trichomonas vaginalis) infect the urogenital tract of humans, (no symptoms in males, but produces vaginitus in females) being passed by sexual contact with an infected individual. Pentatrichomonas hominis lives in the lower bowel of humans, and Trichomonas tenax lives in the human mouth. Other species live in other vertebrate classes and many invertebrates. Phylum Chlorophyta (Volvox) Contains a variety of diverse forms, but members are all autotrophic, containing one or more chloroplasts. It includes flagellated single cell organisms e.g. Chlamydomonas and Volvox, a multicellular colonial form (shown right). Volvox is a green, hollow sphere that reaches 0.5mm to 1mm in diameter. A single organism contains many thousands of individual cells embedded in a jelly ball. Each cell has a nucleus, a pair of flagella, and a large chloroplast and cytoplasm strands connect adjacent cells. Both sexual and asexual reproduction occurs, with daughter colonies being contained within the large parent colony, before it ruptures, releasing them. Sexual reproduction involves the formation of zygotes in the autumn, which are dormant in winter. Phylum Apicomplexa (Plasmodium) All Apicomplexans are endoparasites, with hosts in many animal phyla. Locomotory organelles are less obvious in this group, pseudopodia occur in some stages of the life cycle, and gametes of some species are flagellated. Life cycles usually include both asexual and sexual reproduction and sometimes an invertebrate intermediate host. At some point in their life cycle the organism develop a spore (oocyst), which is infective for the next host and is often protected by a resistant host. One of the best known of the group is plasmodium spp, which is the causative agent of the important infectious disease of humans, malaria. This is a very serious disease, difficult to control and widespread, particularly in tropical and subtropical countries. Four species infect humans, each producing a slightly different clinical set of symptoms. The parasite is carried by mosquitoes (Anopheles) and sporozites are injected into a human with the insect's saliva. The sporozites infect liver cells and proliferate. The period when the parasite is in the liver is the incubation period, and it lasts 6-15 days, depending on the plasmodium species. In the next stage of the organism's life cycle, the amoeboid trophozoites move to infect red blood cells. Release of waste products of these organisms in the blood results in fever and chills in the host, the periods of each being diagnostic of the infecting species. During proliferation in the blood, gametocytes are formed, which when ingested by a feeding mosquito, form zygotes. The zygotes develop into a motile form, which penetrates the insect's stomach wall and further proliferates into sprozites, which migrate to the mosquito's salivary glands allowing for the cycle to be repeated. Elimination of mosquitoes and their breeding grounds by insecticides, drainage and other methods have been effective in controlling malaria in some areas. However, local difficulties including remoteness of location, resistance to insecticides and civil unrest mean that malaria will be a disease for quite some time. Humans have evolved a genetic solution to the problem. The gene for sickle cell anaemia confers a degree of protection from this disease as it interferes with the blood stage of the life cycle. Unfortunately, the gene for sickle cell anaemia causes chronic and lifelong health problems. Sufferers are mostly healthy, but their lives are punctuated by periodic painful attacks. Their life expectancy is shortened to perhaps 40-50 years. Although dreadfully traumatic for the families affected, possession of the gene does confer a genetic advantage, in that individuals possessing the gene in a malaria zone, (in the evolutionary past) lived long enough to reproduce, whereas those who did not, may have died in childhood due to malaria, and not passed any genes onto subsequent generations. Phylum Ciliophora (Paramecium) Ciliates are a large and diverse group, living in all types of freshwater and marine habitats. Indeed, the motile forms make up a large proportion of zooplankton in the sea, the majority of which are free living, solitary, motile organisms, but there are also commensual, parasitic, sessile or colonial species. There is great diversity in shape and size, from 10um 12um to up to 3mm long. All have cilia that beat in a coordinated way, although the arrangement, and possession of cilia in all life stages, varies. Paramecium is a representative ciliate and is abundant in ponds or slow moving streams containing aquatic plants and decaying organic matter. They are often described as slipper shaped, with an asymmetrical appearance due to their oral groove (cytosome), a depression on the ventral side. The pellicle is a clear, elastic membrane covered in cilia arranged in rows lengthwise. Along the cytosine are more cilia, which keep food particles moving down until they form food vacuoles. Non-digested material is usually released from the same area of the cell. Paramecia are holographic, living on bacteria, algae and other small organisms. When Paramecia comes into contact with a barrier, or a disturbing chemical stimulus, it reverses its cilia, backs up a short distance and swerves the anterior end, as it pivots on its posterior end. This behaviour is known as an avoiding reaction. The same mechanism is used to keep the organisms within the range of an attractant. A paramecium may also alter its swimming speed. These alterations in locomotion are thought to be initiated by changes in potential difference in the cell membrane caused by the stimulus. Locomotary responses by any organism are termed taxes (sing. taxis). Movement towards a stimulus is termed a positive taxis, those away from a negative stimulus a negative taxis. For example: Thermotaxis - response to heat Phototaxis - response to light Thigotaxis - response to contact Chemotaxis - response to chemical substances Rheotaxis - response to currents (air or water) Galvanotaxis - response to constant electrical current Geotaxis - response to gravity Paramecia reproduce by binary fission, but also have forms of sexual phenomena, e.g. conjugation (union of two individuals, which exchange genetic material) and autogamy (a process of self-fertilisation). Phylum Dionflagellata About half of this group has chromatophores, bearing chlorophyll; ecologically some species are among the most important primary producers in marine environments. A distinguishing feature is the presence of two flagella, and individuals may have protective cellulose plates. Some dinoflagellates live in mutuality association in tissues in certain invertebrates, including other protozoa, sea anemones, horny and stony corals and clams. The association with stony corals is significant, because only corals with symbiotic dinoflagellates of the zooxanthellae can form coral reefs. Dinoflagellates can cause significant harm to other organisms, such as when they produce a 'red tide'. This is when an unusual proliferation causes high levels of toxins to occur. The water may appear red, brown, yellow, or not coloured at all. The toxins are apparently not harmful to the dinoflagellates that produce them, but can be highly poisonous to fish and other forms of marine life. Amoebas Members of this group do not form single phyla, but for simplicity due to other common features, they will be discussed together. Amoebas are found in soil in both fresh and marine waters. Some are planktonic, others prefer to live on a surface, and a minority are parasitic. Nutrition in these organisms is holozoic, i.e. they ingest and digest liquids or solid food. Most are omnivorous, consuming bacteria, protozoa, rotifers and other microscopic organisms. Amoebas take in food at any point in their bodies by extending pseudopodia and engulfing the particle (phagocytosis). Most organisms in the group reproduce by binary fission, although budding and sporolation can occur in some species. The most common studied species is Amoeba proteus, which can have a variable appearance. It lives in slow streams and ponds of clear water (rarely found in free water), often on aquatic vegetation on the sides of ledges. They are colourless and about 250um and 600um in diameter. Organelles such as nucleus, contractile vacuoles and food vacuoles can be clearly seen with a light microscope. Individuals can survive for several days without food, but it decreases in volume during starvation. There are several species in this group, which live in the intestines of humans and other animals, both vertebrate and invertebrate. Forminiferans are an ancient group of shelled Amoebas found in all oceans, with a few in fresh or brackish waters. Most of this group live on the ocean floor, and are numerous, constituting arguably the largest biomass of any animal group on Earth. The foraminiferal life cycle involves an alternation between haploid and diploid generations, although both are usually similar in form. The haploid or gamont initially has a single nucleus, and divides to produce numerous gametes, which typically have two flagella. The diploid or schizont is multinucleate, and after meiosis, fragments to produce new gamonts. Multiple rounds of asexual reproduction between sexual generations are common in benthic forms. The form and composition of the test (shell) is the primary means by which forams are identified and classified. Most have calcareous tests, composed of calcium carbonate. It can also be composed of organic material, made from small pieces of sediment cemented together (agglutinated), and in one genus of silica. Openings in the test, including those that allow cytoplasm to flow between chambers, are called apertures. Tests for fossils date as far back as the Cambrian period, and many fossil forms resemble closely those found today. They were especially abundant in the Cretaceous and Tertiary periods, and were amongst the largest protozoa ever to exist, measuring up to 100mm or more in diameter. Of practical importance are the remains of foraminiferous, which form limestone and chalk deposits. For example, the limestone that makes up the pyramids of Egypt is composed almost entirely of nummulitic benthic foraminifera, as are the White Cliffs of Dover. Identification of fossil species in test drillings is also useful to oil geologists for identifying the age of rock strata and the possibility of oil deposition. Multicellularity, Mesozoa and Parazoa Discovering the origin of multicellular animals has been problematic for zoologists and there are three main hypotheses. 1. Metazoans (multicellular animals) arose from a syncytial (multinucleate) ciliated form in which cell boundaries later evolved. In this theory, it is presumed that the body form of the ancestor resembled modern ciliates and was bilaterally symmetrical, leading to bilaterally symmetrical metazoans, like the present flatworms. Objections for this hypothesis include the fact that it ignores the embryology of flatworms, in which nothing resembling this cellularisation occurs. It also cannot account for flagellated sperm in metazoa, and, more importantly, it implies that the radial symmetry of cnidarians is derived from a primary bilateral symmetry, for which there is no evidence. 2. They arose from a colonial flagellated form in which cells gradually became more specialised and interdependent. The colonial flagellate hypothesis, although first proposed in 1874 (Haeckel), still has many followers, although it has undergone revisions. It proposes that multicellular animals descended from ancestors, which were a hollow spherical colony of flagellated cells. Individual cells within the colony became specialised for specific roles (reproductive cells, nerve cells, somatic cells etc). Thus, cellular independence was lost in favour of the welfare of the colony as a whole. From this perspective, the colonial ancestral form was at first radically symmetrical, perhaps similar to the panula larvae of the cnidarians (jellyfish, etc). This larva is radially symmetrical with no mouth, and cnidarians could not have evolved from another form. Bilateral symmetry could have evolved later when some of these planula-like ancestors became adapted for a creeping form of locomotion on the ocean floor.