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Noncoelomate AnimalsI. The Classification Of AnimalsA. Divided into Two Subkingdoms 1. Parazoa a) Lack definite symmetry b) Neither tissues nor organs are present c) Consist primarily of sponges 2. Eumetazoa a) Possess definite shape and symmetry b) Have tissues organized into organs and organ systems c) Include all other animals, 35 phyla B. Comparison of Parazoa and Eumetazoa 1. Eumetazoa are generally more complex than sponges 2. Eumetazoa form three distinct embryonic cell layers a) Outer ectoderm, inner endoderm and intermediate mesoderm b) Layers differentiate into tissues of the adult c) Sponges lack tissue layers 3. Evolutionary relationships a) Derived from the same unicellular ancestor b) Resembled choanoflagellate protists C. Subgrouping of Invertebrate Animals 1. "Primitive" or "lower" invertebrates a) Have less complex tissue organization b) Comprise 14 phyla c) Four phyla more successful than the others d) Porifera = sponges (1) Cnidaria = jelly fish + hydroids + sea anemones + corals (2) Platyhelminthes = flatworms (3) Nematoda = roundworms 2. "Advanced" or "higher" invertebrates II. Phylum Porifera: The Sponges A. General Biology of Sponges 1. Primarily marine species, fewer freshwater varieties 2. Few radially symmetrical, but most lack any symmetry 3. Many are colonial, all are sessile as adults 4. Cellular organization a) Little coordination among cells b) Simple mass of cells in a gelatinous matrix c) Cells are specialized for different functions 5. Filter feeders a) Water flows through system of pores and canals b) Water forced out through a larger pore called the osculum c) Water forced through passageways by beating flagella 6. Basic structure a) Choanocytes (1) Specialized flagellated cells that face inward (2) Line internal cavity or specialized chambers in large sponges b) Epithelial layer of flattened cells, frequently contractile in nature c) Mesenchyme (1) Intermediate gelatinous layer with amoeboid cells (2) May possess minute, needles called spicules (3) May possess fibrous spongin protein network B. The Choanocyte 1. Structurally resembles a protist with a single flagellum a) Independent beating of flagella creates water currents b) Used to acquire food and oxygen and expel wastes c) Body cavity inner wall may be convoluted to increase surface area 2. Microstructure a) Base of flagellum surrounded by collar of hair-like projections b) Strands of collar connected by microfibrils c) Food particles in water filtered by collar, collected in mucus d) Food digested by collar cell or adjacent amoeboid cell C. Reproduction in Sponges 1. Frequent reproduction by fragmentation 2. Sexual reproduction via production of egg and sperm a) Larval sponges undergo development within adults b) Have external choanocytes when released c) Exist as free-swimming planktonic form for a short time d) Settle on a suitable substrate to begin a sessile adult life e) Turn inside out, choanocytes become internal III. Phylum Cnidaria: The Cnidarians A. Two Phyla Exhibit Radial Symmetry Throughout Life 1. Cnidaria: hydroids, jellyfish, sea anemones and corals 2. Ctenophora: comb jellies 3. Both are considered to be most primitive Eumetazoans 4. Primitive since tissues not organized into organs 5. Were widespread in Precambrian times B. General Biology of Phylum Cnidaria 1. Nearly all are marine, only a few are freshwater 2. Basically gelatinous, have tissues but no organs 3. Carnivores, capture food with tentacles that surround mouth 4. Exhibit two body forms a) Polyp: cylindrical, generally attached to a substrate (1) Solitary or colonial (2) Mouth faces away from substrate, generally upward (3) May form hard internal or external skeleton b) Medusa: umbrella-shaped, free-floating (1) Mouth faces substrate, generally downward (2) Possess a thick jelly-like mesoglea, between epidermis and gastrodermis c) May exist in polyp or medusa forms only or alternate between the two phases d) Both forms are diploid 5. Reproduction a) Polyps reproduce asexually by budding, form polyps or medusae b) Sexual reproduction produces fertilized eggs c) Develops into a free swimming, multicellular, ciliated planula larva 6. Evolutionary advancement: development of an internal digestive cavity a) Digestive enzymes secreted into a primitive gut b) Food broken into smaller particles c) Particles further digested by cells lining gut d) Enable cnidarians to digest food larger than individual cells e) Undigested food particles exit gut through mouth f) Gastrovascular cavity = gut = coelenteron: has only one opening 7. Organization of tissues a) Nerve cells organized into nets to coordinate muscle contraction b) No blood vessels c) No respiratory system d) No specialized internal cavity 8. Cnidaria possess cnidocytes a) Structures specialized for food capture and defense b) Located on tentacles, sometimes the body surface c) Each cnidocyte contains a harpoon-like nematocyst d) Propelled by water pressure e) Protein toxin injected into prey (1) Portuguese man-of-war possesses powerful neurotoxins (2) Stings of other jellyfish can be severely painful 9. Symbiotic relationships of nudibranchs and flatworms a) Ingest nematocysts without discharge b) Retain nematocysts for own defense systems C. Classes of Cnidarians 1. Class Hydrozoa: hydroids a) Have both polyp and medusa forms b) Mostly marine, colonial forms c) Example: Portuguese man-of-war d) Example: freshwater Hydra (1) Atypical, has polyp form only (2) Readily glides on basal disk or somersaults 2. Class Scyphozoa: jellyfish a) Conspicuous medusae alternate with inconspicuous polyp forms b) Outer layer contains contractile epitheliomuscular cells c) Separate male and female individuals produce planulae 3. Class Anthozoa: sea anemones and corals a) Solitary and colonial marine organisms b) Cylindrical body with tuft of tentacles in multiples of six c) Live primarily in shallow warm waters, harbor photosynthetic algae d) Exclusively polyp form e) Sea anemones are soft-bodied f) Corals secrete hard or protein skeletons that comprise coral reefs IV. Phylum Ctenophora: The Comb Jellies A. Relationship to Cnidarians 1. Traditionally thought to be closely related 2. Recent research questions this assumption B. General Biology of the Ctenophorans 1. Transparent spherical to ribbon-shaped forms, few centimeters long 2. More complex than Cnidaria, have anal pores 3. Abundant in the open ocean 4. Have two long retractable tentacles 5. Possess eight comb-like plates of fused cilia for locomotion 6. Many are luminescent V. Bilateral Symmetry A. Comparison of Bilateral Symmetry to Radial Symmetry 1. Bilateral organisms exhibit right and left halves a) Possess dorsal (to p) and ventral (botto m) halves b) Differentiate anterior (fron t) and posterior (bac k) 2. Allows for differential adaptation of various parts of body B. Body Plans of Bilaterally Symmetrical Animals 1. Acoelomate: possess no body cavity other than digestive system 2. Pseudocoelomate: body cavity develops between mesoderm, endoderm 3. Coelomate: body cavity completely bounded by mesoderm, internal organs suspended in it by mesenteries 4. Examples a) Acoelomates and pseudocoelomates are all worms b) Coelomates are more diverse in body form, include the vertebrates VI. ACOELOMATES: THE SOLID WORMS A. Phylum Platyhelminthes: The Flatworms 1. General biology a) Dorsoventrally flattened bodies, have definite head at anterior end b) Bodies are solid, gut is the only internal cavity c) Many species are parasitic others species are free-living carnivores or scavengers d) Move via ciliated epithelial cells on lower surface 2. Organ systems of flatworms a) Digestive system is branched with a single opening (1) Cannot feed continuously (2) Gut also functions to transport food (3) Partial extracellular digestion, also phagocytosis (4) Tapeworms lack digestive system, bathed in nutrients b) Excretory system of fine tubules with bulb-like flame cells (1) Primarily regulate water balance (2) Excretion evolved secondarily c) Lack a circulatory system, food and oxygen transported via diffusion d) Simple nervous system with longitudinal nerve cords, primitive brain e) Free-living forms possess sensory systems on side of head (1) Detect chemicals and fluid movements associated with food (2) Eyespots are light sensitive, pigmented cups f) Generally more active than radially symmetrical invertebrates g) Reproductive systems are complex (1) Most flatworms are hermaphroditic with internal fertilization (2) Asexually reproduce by fragmentation followed by regeneration 3. Class Turbellaria: the turbellarians a) Free living organisms, found in water and moist habitats b) Example: Dugesia, the common planarian 4. Class Trematoda: the flukes a) Parasitic forms have epithelium and gut linings resistant to host digestive enzymes b) Lack sensory and locomotive adaptations of free-living forms c) Take food in through mouth d) Have complex life cycles involving one, two or more hosts e) Example: Clonorchis sinensis, human liver fluke (1) Eggs containing miracidium larva passed out in feces (2) Ingested by snail, transformed into sporocyst (3) Sporocysts produce rediae (4) Nonciliated redia give rise to cercariae (5) Cercariae released in water, are free-swimming (6) Bore into muscles of fish, turn into metacercariae (7) Humans eat fish, cysts dissolve, flukes migrate to liver f) Example: Schistosoma blood flukes (1) Life cycle a) Eggs leave body in urine and feces, hatch into miracidia b) Become sporocytes which form daughter sporocytes c) Daughter sporocytes become cercariae, burrow into human skin d) Get into blood stream, go to lungs, go back to blood e) Male and female worms mate on way to hepatic and portal veins f) Adults live in blood vessels feeding intestines or bladder g) Shed eggs continuously for many years (2) Disease schistosomiasis is spreading through the tropics a) Control via breaking life cycle b) Investigations into effects on immune reaction, develop vaccine 5. Class Cestoda: tapeworms a) Extremely specialized parasitic organisms b) Absorb food through outer body wall c) Bodies divides into scolex, neck and reproductive proglottids (1) Proglottids formed continuously from region behind neck (2) Eggs toward end mature, become fertilized (3) Embryos emerge from end proglottids, leave host in feces d) Example: Taenia saginata, beef tapeworm (often found in humans) B. Phylum Rhynchocoela: The Ribbon Worms 1. Mostly marine, free-living ribbon-shaped or thread-shaped worms 2. Have a long, muscular, retractable proboscis for capturing prey 3. Simplest organisms that possess complete digestive system 4. Simplest animals with closed circulatory system VII. A Body Cavity A. Pseudocoelomate Animals 1. Include seven phyla, Nematoda contains the most members 2. Pseudocoel serves as a hydrostatic skeleton against which muscles contract 3. Lack defined circulatory systems 4. Have complete, one-way digestive tract B. Phylum Nematoda: Nematodes 1. Include nematodes, eelworms and roundworms 2. Ubiquitous and abundant in marine, freshwater and terrestrial habitats 3. Most are microscopic in size, parasitic and live in soil 4. General biology of nematodes a) Bilaterally symmetrical, cylindrical, unsegmented worms b) Covered by thick, flexible cuticle that is molted periodically c) Have longitudinal muscles located beneath the epidermis (1) Pull against cuticle and pseudocoel (2) Results in side-to-side whipping movement d) Specialized digestive system with piercing stylets, mouth, pharynx and anus e) Completely lack cilia or flagella f) Excretory systems of canals or glands not dependent on cilia g) Reproduction sexual, generally the sexes are separate h) Development is simple, precise (1) Caenorhabditid elegans composed of 1000 cells (2) Fate of each cell completely mapped out C. Many Nematodes Parasitize Humans 1. Example: Trichinella, pig intestinal roundworm a) Trichinosis may occur if pork eaten raw or undercooked b) Worms may also infect bears and be transmitted to humans 2. Example: Filaria, worms living in lymphatic vessels a) Infection passed by mosquitos b) Causes gross swelling of extremities, elephantiasis D. Phylum Rotifera: Rotifers 1. Microscopic animals found in aquatic and soil habitats 2. Have crown of cilia at heads for feeding and locomotion 3. Have muscular pharynx with grinding jaws inside 4. Have flame cells like flatworms to control osmotic pressure 5. Sexual reproduction with separate sexes 6. Some species possess only females and reproduce solely by parthenogenesis Layout by J.T. Poirier © 2001 |