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 320Invertebrate ZoologyFall 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 • Major milestone - cephalization • Therefore, has allowed the colonization of more physiologically challenging habitats • 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 • Known as midsaggital plane • Produces left and right mirror-image halves • Body polarized along two axis • 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 • Remember, smooth muscle contracts slowly • Outer layer of circular muscle almost always encloses an inner longitudinal layer • Especially true in vermiform animals • Contraction of circular muscles produces elongation • Due to hydrostatic skeleton
Longitudinal muscle contractions allow for: • Bending • Undulating • Shortening / retraction of posterior end • Peristalsis • 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 • 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 • 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) • Fluid is known as coelomic fluid • Circulated by mesothelial cilia or muscular contractions • Functions • 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 • Protostomia – molluscs, annelids, and arthropods • Deuterostomia – echinoderms and chordates • Protostomes • Spiral cleavage • Schizocoely • Blastopore becomes mouth • Deuterostomes • Radial cleavage • Enterocoely • Blastopore becomes anus, and mouth forms elsewhere