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Practice Questions for Exam 1 Reproduction that occurs when a portion of a parent splits off to form a new individual is a. Asexual b. Budding c. Sexual d. a and b e. None of the above Which one of the following regarding the protozoans is false? a. They are often colonial b. Some have chloroplasts and photosynthesize c. Some possess flagella for locomotion d. They are metazoan e. Some reproduce by fission Biology 320 Invertebrate Zoology Fall 2005 Chapter 9 – Introduction to Bilateria Bilateria Majority (99%) of eumetazoans exhibit bilateral symmetry These organisms are lumped into the taxon Bilateria Has led to the enhancement of neuromuscular systems Therefore, has allowed the colonization of more physiologically challenging habitats Major milestone - cephalization In particular, terrestrial habitats Also allowed the attainment of larger body sizes 90m in the blue whale Bilateral Symmetry Animal has only one plane of bisection Body polarized along two axis Known as midsaggital plane Produces left and right mirrorimage halves Anterior / posterior (head / tail) Ventral / dorsal (belly / back) Most likely evolved from organisms who began spending time at air / water interface Bilateral symmetry is most often seen in animals that move horizontally through habitat Useful because food / mates usually randomly distributed Sessile animals often filter feed Radial symmetry is suitable to lifestyle Plankton / particulate matter is somewhat uniformly distributed Cephalization Means “head development” Anterior localization of CNS and sensory structures Brain usually one or more ganglial complexes Vertebrates have skull and spinal column to protect large CNS Exhibited by motile bilateria Allows motile animals to detect and pursue food, mates, shelter, etc. Mouth at anterior end Longitudinal nerve cords Usually paired Often have giant axons for rapid conductance (low resistance), presumably for escape response To contrast motile bilateria, sessile bilateria usually are not cephalized May have radially symmetric portions such as crown of tentacles Musculature As many are motile, often have obliquely / cross striated muscle for rapid contraction Outer layer of circular muscle almost always encloses an inner longitudinal layer Remember, smooth muscle contracts slowly Especially true in vermiform animals Contraction of circular muscles produces elongation Due to hydrostatic skeleton Longitudinal muscle contractions allow for: Peristalsis Bending Undulating Shortening / retraction of posterior end Coordinated / alternating contractions of circular and longitudinal muscles Useful for forward propulsion and burrowing Other types of musculature Dorsoventral – flattening Helical – twisting Bilaterian Compartmentalization Important for specialization of physiological regulation Cnidarians first evolved a cavity for extracellular digestion However, the gastrovascular cavity is not specialized Carries out multiple functions Example, bilaterian gut specializes in digestion and absorption Bilaterians have evolved other body systems that increase physiological compartmentalization Coelom Hemal system The Bilaterian Gut More primitive bilaterians still posses a blind gut Many have a true gut (hollow tube) that is composed of specialized regions Mouth and foregut – ingestion and initial digestion Midgut – chemical digestion and absorption Hindgut and anus – elimination of wastes, reclamation of water, and ion regulation Origins of gut regions Foregut and hindgut develop from embryonic ectoderm Midgut develops from embryonic endoderm; becomes gastrodermis Specialized structures of gut regions Foregut Midgut Buccal cavity – chamber that receives food and may house teeth Pharynx – throat that may be protrusible in some animals Esophagus – ciliated portion that links foregut and midgut Stomach – enlarged for digestion Intestine – forms feces and joins hindgut Ceca – outpockets of stomach or intestine that increase SA for digestion, absorption, and/or storage Hindgut Rectum – receives indigestible wastes Cloaca – name for rectum if gonoducts / excretory ducts empty here Keep in mind, names of gut regions and structures are not standardized across the phyla! Coeloms Fluid-filled cavity lined with mesodermally derived epithelium (mesothelium) Functions Fluid is known as coelomic fluid Circulated by mesothelial cilia or muscular contractions Houses organs Hydrostatic skeleton Circulation Reproduction Excretion Three types Acoelomates – typical of small animals as they are not diffusion limited Pseudocoelomates – cavity not entirely lined with mesothelium Coelomates – true body cavity lined with mesoderm-derived mesothelium or peritoneum Segmentation occurs when coelom is partitioned by: Mesenteries – longitudinal Septa – transverse Benefit is that each segment can be regulated individually Locomotion Loss of some segments may not be life threatening Hemal System Essentially a circulatory system that consists of: Blood (called hemolymph in most inverts) Vessels Sinuses Blood is usually circulated via muscular contractions Hemal system is usually absent in small animals Some organisms (especially pseudocoelomates) have a hemocoel Large circulatory sinus that doubles as main body cavity Excretion Larger animals have nephridia (little kidneys) Remove nitrogenous waste and osmoregulate Wastes are removed w/ water out of a nephridiopore Variation in nephridia Cleavage Patterns Diagnostic tool for systematics Radial cleavage Cleavage planes are parallel or perpendicular to axis of zygote Occurs in cnidarians, lophophorates, and deuterostomes (covered next) Spiral cleavage Cleavage planes are oblique to axis of zygote Occurs in flatworms, molluscs, segmented worms, etc. Coelom Formation Enterocoely Outfoldings of archenteron pinch off Occurs in deuterostomes Schizocoely Coelom splits early in development Forms segmental coelomic cavities Divisions of Bilateria Two major divisions Protostomes Protostomia – molluscs, annelids, and arthropods Deuterostomia – echinoderms and chordates Spiral cleavage Schizocoely Blastopore becomes mouth Deuterostomes Radial cleavage Enterocoely Blastopore becomes anus, and mouth forms elsewhere