Bilateral Acoelomates

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Chapter 14 Bilateral

Acoelomates

Acoelomate

• No internal body space other than digestive tube.

• Spaces between organs filled with a loose connective tissue with more cells than mesoglea = parenchyma.

• Solid construction is a diagnostic character of the phylum.

General Features

• Cephalization – Sessile animals survive well with radial symmetry

– Concentrating the sense organs on the head is an advantage to active animals that seek food

– This provides an anterior and posterior end and bilateral symmetry

Development of a Centralized Nervous System

• Protozoa are unicellular and lack nerves • Nerve Net

– This is the simplest pattern of nervous system found in sea anemones, jellyfish, hydra and comb jellies.

– The nerve net is an extensive network in and under the epidermis.

– c.Impulses starting in one part are conducted in all directions; synapses do not direct one­way impulses.

– There are no sensory, motor or interneurons.

– This type of system survives in advanced animals as a nerve plexus that governs intestinal movement.

CNS continued • Bilateral Nervous Systems

– Flatworms represent the simplest bilateral nervous system.

– They have two anterior ganglia leading to two main nerve trunks that run posteriorly.

– Lateral branches form a ladder appearance.

– This is the simplest system to have a peripheral nervous system extending to all parts of the body, and a central nervous system concentrating nerve cell bodies.

Position and Biological Contributions

• Simplest animals with primary bilateral symmetry

• Solid body without a coelom • Organ­system level of organization • Protostomes with spiral cleavage • Triploblastic with well defined mesoderm • Simple nervous, excretory, and osmoregulatory systems appear

Phylum Platyhelminthes

General Characteristics

• Dorsoventrally flattened animals with primary bilateral symmetry.

• Without coelom, anus, circulatory, respiratory, or skeletal systems.

• With flame­bulb protonephridia. • With a cellular mesodermal connective tissue filling all spaces between organ­ systems.

• The phylum includes the free living flatworms and the parasitic flukes and tapeworms.

Homeostasis: Water and Osmotic Regulation

• Invertebrates Meet Problems of Salt and Water Balance – Most marine invertebrates are in osmotic equilibrium with their seawater environment.

– With body surfaces permeable to water and salts, the internal and external concentrations are equal.

– Such animals that cannot regulate osmotic pressure of their body fluids are called osmotic conformers.

– This functions for open ocean organisms because the open ocean is stable.

Water and Osmotic Regulation

• Animals that must live within a narrow salinity range are stenohaline.

• Organisms that can tolerate the wide variations found in estuaries are euryhaline.

• A hyperosmotic regulator maintains body fluids in higher concentration than surrounding water. – Maintaining a higher concentration by excreting excess water requires specialized excretory organs

Protonephridium • This tubular structure is the

most common design to maintain osmotic balance.

• The flame cells system or protonephridium is the simplest arrangement.

• Planaria and other flatworms have a highly branched duct system to all parts of the body.

• Fluid enters the system through specialized “flame cells” and passes through tubules to exit the body.

Protonephridia • Rhythmical beating of a flagellar tuft creates a negative pressure that draws fluid into the tubes.

• In the tubule, water and metabolites are recovered by reabsorption; wastes are left to be expelled.

• Nitrogenous wastes, mainly ammonia, diffuse across the surface of the body.

• Flatworms have no circulatory system so the flame cell system must branch throughout the animal.

• This system is a closed system since the fluid must pass across flame cells.

Bilateral Symmetry

• All remaining animal phyla are bilaterally symmetrical.

• Correlated with motility. Anterior end meets environment first and bears most of the sense organs.

• Cephalization results. – Right and left sides each contact environment in same way so virtually identical, but upper and lower halves differ as do front and back.

• Of all bilaterally symmetrical animals these are probably the most primitive.

Germ Layers • Three distinct germ layers; Ectoderm (outer), (endo)mesoderm(middle) and endoderm (inner).

• Triploblastic). • Ectodermà surface layer (skin, hair, etc.) And nervous system.

• Endoderm à gut lining and internal organs. • Mesodermà everything else (muscles, blood, bones, connective tissue, etc.).

Appearance

• Range in size from microscopic worms no larger than protozoans to the extreme elongation seen in tapeworms which can be 10 to 15 m, but most are small to moderate size.

• Most are white or colorless or owe their color to the food they ingest; Free­living worms are often gray, brown or black (when not white), but some are brightly colored.

Appearance

• May have clearly recognizable head at anterior end separated from body by neck­like constriction, but anterior is organized as a head even when the constriction is absent.

• Can tell it is head because the main nerve center is there, the presence of numerous sensory organs, and because it is directed forward during locomotion.

• In parasitic flatworms head usually bears hooks and suckers to attach to host (but these may be elsewhere on body also).

Appearance

• Ventral surface on which locomotion occurs (often ciliated), and which also bears mouth and genital pores, differs from dorsal surface in turbellarians.

• Difference is less marked in trematodes and virtually absent in cestodes.

Body Wall

• Ciliated epidermis one layer thick in f.L. For locomotion.

• Tegument in parasites. – Cells of tegument lie under muscle layers and extend up as a syncytial layer of cytoplasm over surface.

• Beneath surface layer generally find circular, longitudinal and often also diagonal muscle fibers of mesodermal origin.

Hydrostatic Skeleton

• Many invertebrates use their body fluids as an internal hydrostatic skeleton.

• Muscles in the body wall contract against the coelomic fluids that are incompressible.

• In flatworms, if body fluid is lost, the worm cannot continue to move. – Annelids “solved” this problem by dividing the body into compartments by septa so that each surviving part can still develop pressure and move.

Nervous System

• Nervous system of the most primitive flatworms derivable directly from cnidarians. = Epidermal nerve net and a slight massing of nervous tissue representing a brain.

• In most, the nervous system has sunk below epidermis and muscle layer and the main nerve net shows concentration into several longitudinal ganglionated cords ­ 2 ventral ones are either most conspicuous ones or only ones.

Nervous System

• System primarily radial becomes bilateral by elimination of some of the radial cords (around bell of medusa).

• Brain = main nervous center. • Ladder type of nervous system when only two nerve cords present because joined periodically by transverse connections.

Sense Organs • Stimuli

– Sense organs are specialized receptors designed to detect environmental status and change.

– Sense organs are the first level of perception and are channels for bringing information to the brain.

– A stimulus is some form of energy: electrical, mechanical, chemical or radiant.

– The sense organ are biological transducers that must transform the energy form of the stimulus into nerve impulses. • usually respond to only one kind of stimulus.

– Muller (1830s) detected that animals perceived different sensations only because impulses originating from one sense organ arrive at a particular sensory area: the “law of specific nerve energies.”

Chemoreception

• This is the oldest and most universal sense in the animal kingdom.

• Protozoa use contact chemical receptors to locate food and oxygenated water, and to avoid harmful substances.

• Chemotaxis is an orientation behavior toward or away from a chemical source.

• Most metazoans have specialized and sensitive distance chemical receptors or a “sense of smell.”

Specific Sense Organs

• Turbellarians richly supplied with sensory organs ­ greatly reduced in parasitic classes chemoreceptors ­ widespread. – tangoreceptors (for sensing touch) widespread. – statocysts in some. – ocelli ­ widespread. Also in larval flukes and some adult groups.

Digestive System

• Similar to anthozoans. Has mouth, pharynx and blind intestine, no anus.

• Mouth is located at about the center of the ventral surface. Primitive.

• Pharynx varies in phylum. Usually is a strong muscular tube which can sometimes be extruded.

• Intestine varies from a simple sac to complicated branchings; Missing completely in some primitive turbellarians and in cestodes (tapeworms). Can you think of a reason why tapeworms wouldn’t need an intestine?

Reproduction

• Reproductive system is most complicated system (in tapeworms there is little else).

• Almost all are monoecious. • Reciprocal fertilization is accomplished by a complicated copulatory apparatus.

Tapeworm Reproduction

Figures 14.21 and 14.22

Reproduction

• Fertilization is internal. – Fertilized, shelled eggs, often already containing embryos are shed to exterior or fastened to objects by adhesive secretions.

• Asexual fission in some turbellarians. Reduced population density triggers asexual strategy.

• Asexual reproduction in many parasitic stages. Important mechanism to increase reproductive potential.

Ecology

• Either free­living or ecto­or endocommensals or parasites.

• Class Turbellaria are mainly free living. • Classes Trematoda and Cestoda are wholly parasitic, chiefly endoparasitic. – Adult stage parasitizes vertebrates (exceptions rare). – Larval stages often occur in invertebrates, usually snails.

– Hosts may be fresh­water, marine or terrestrial.

Turbellaria

• Free­living flatworms. Live in fresh water, in springs, streams, ponds and lakes; Or in the ocean, chiefly along shores; Or in moist terrestrial habitats, mostly in tropical or subtropical regions. – A few Turbellarians are commensal or parasitic.

Trematoda • All parasitic flukes • Many are serious parasites in humans. • Complicated life cycle with at least 2 different hosts.

• Tegument lacks cilia in adults • Adaptations for parasitism include:

– Penetration glands – Glands to produce cyst material – Hooks and suckers for adhesion – Increased reproductive capacity

• Sense organs are poorly developed

Cestoda

• Segmented entoparasites in digestive tract and associated ducts of vertebrates.

• Have no epidermis, mouth or digestive tract.

Cestode Morphology.

• 3 body regions. • Scolex (holdfast organ) located at the anterior end and provided with suckers and/or hooks for adhering to the host. Can be quite complicated. Small when compared to a mature proglottid.

Cestode Morphology

• Neck region. • Situated immediately posterior to scolex. • Unsegmented, poorly differentiated area generally narrower than scolex and strobila proper.

• Continuously differentiating zone which gives rise to new proglottids (body segments).

Cestode Morphology.

• Strobila. • Main bulk of body. • Linearly arranged segments called proglottids form a chain.

• Newest segments at neck and oldest at tail end.

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Cestode Morphology (Segmentation)

• Metamerism (segmentation) is a serial repetition of body parts along anterior­posterior axis.

• Differs from "typical" segmentation (metamerism) of annelids, arthropods and chordates.

• If above definition is true, this is segmentation. – Big difference is in direction. – In cestodes go from scolex ­­> young segments. ­­> Old segments. In others get new segments from segment before end.

Cestode Body Wall

• Several layers thick. • Outermost is tegument (nonciliated cytoplasmic syncytium overlying muscle layers).

• The syncytium is formed from extensions of cells located in the parenchyma.

• Differs from tegument of trematodes in having surface microthrix (= microvilli; Surface folds) which increase the surface area through which food can be absorbed. Pinocytosis and exocytosis (excretion) can occur here.

Other Cestode features

• Respiration is largely anaerobic, but have enzymes for aerobic respiration also.

• Nervous system is typical, but no specialized sense organs.

• Reproduction­ mono and dioecious.

Reproduction

• Monoecious. Self fertilization within single proglottid and between proglottids, and copulation between 2 worms if they bump into each other in the gut of the host all occur in monoecious.

• Selfing between different proglottids of the same worm is probably deleterious. When one worm per mouse (Hymenolopus diminuta) found an increase in frequency of abnormal larvae. Also a decrease in egg viability. No selfed strain can be maintained beyond 5th generation so it is probable that cross fertilizaion to provide hybrid vigor is essential at least occasionally. Usually male organs mature before female so fertilization of same segment is not possible.

Reproduction

• Dioecious. Cross fertilization is necessary. Only one dioecious order.

• Complete reproductive system in each proglottid. Basic structure similar to digenetic trematodes.

• Fertilized eggs usually stored in blind (no exit) uterus. Terminal proglottids packed with eggs break off and eggs are freed and pass out through intestine in feces or proglottids.

Life cycles

• Usually at least 1 or 2 intermediate hosts (arthropods and vertebrates).

• "Typical" life cycle. Egg ­­> shelled larva ­­> cysticercus ­­> adult.

Life cycles

• Larva penetrates intestinal wall and reaches coelom, liver, spleen, muscles or other parts while gradually developing scolex with adhesive organs typical of the species and have young tapeworm, but no strobilation. Encysts in host tissue until host eaten by final host.

• In intestine of final host larva attaches to the wall and develops into mature tapeworm.

• Produces many proglottids each day.

Life Cycles

• Cysticercus. Some use a vertebrate host instead of arthropod to eat larvae and grow cysticercus = bladder worm.

• Humans can eat and get adult tapeworms. Usually from raw or incompletely cooked pork.

• Can get tapeworms from beef as well. Adult tapeworms not too bad. Symptoms absent or mild.

• Much more serious to eat embryonated eggs of pork or beef tapeworm. Bladder worms develop in various organs, including the brain. Symptoms similar to tumor, and can get serious reactions to liberated toxins.

Phylum Nemertea

• Ribbon worms • Long muscular proboscis • General body plan similar to Turbellarians • Ciliated epidermis with many gland cells • Excretory system with flame cells • Mostly dioecious • Mouth to anus digestive system • Most free­living, few commensal or parasitic