Worms Chapter 27
Flatworm characteristics • Soft, flattened bodies • Simplest animals with three embryonic germ layers • Bilateral symmetry • Cephalization: head formation • Acoelomate: fluid filled body cavity made of mesoderm tissue
Flatworm feeding styles • Carnivorous: feed on tiny aquatic animals • Scavengers: feed on recently dead animals • Parasites: blood, tissue fluids, cells within host’s body • Lack complex digestive system because host partially digests food • Ex: tapeworm (no digestive system at all)
Feeding • Mouth only • Pharynx: muscular tube, extended from mouth • Digested food and nutrients absorbed in the digestive cavity
Branched digestive system translucent species Aquaplana/Paraplanocera sp. extends throughout the entire body Photo by Robert F. Bolland).
Respiration, Circulation, & Excretion • Flat and thin body shape requires little circulation. • Diffusion transports oxygen and nutrients to internal tissues • Diffusion removes CO2 and waste • Flame cells: specialized cells that remove excess water from body • helps to remove metabolic waste (ammonia and urea)
Response • Ganglia: group of nerve cells with two long nerve cords • Eyespots: group of cells that can detect changes in light intensity
The Anatomy of a Flatworm Section 27-1 Eyespot Freshwater flatworms have simple ganglia and nerve cords that run the length of the body. The excretory system consists of a network of tubules connected to flame cells that remove excess water and cell wastes. Ganglia Head Nervecords Gastrovascularcavity Flatworms use a pharynx to suck food into the gastrovascular cavity. Digested food diffuses from the cavity into other cells of the body. Eyespots in somespecies detect light. Excretorysystem Ovary Testes Mouth Pharynx Most flatworms are hermaphrodites, having male reproductive organs (testes) and female reproductive organs (ovaries) in the same organism. Flame cell Excretorytubule Figure 27–3
Movement • Cilia on epidermal cells • Nerve controlled muscle cells Muscle filament staining of a turbellarian flatworm reveals a meshwork of longitudinal, circular and diagonal muscles. The large, bright ring with muscle fibers radiating outward is the muscular pharynx plicatus Image by M. D. Hooge and S. Tyler, Department of Biological Sciences, University of Maine).
Groups of flatworms • Turbellarians: free living, marine & freshwater • Flukes: parasitic, infects internal organs of host • Tapeworms: parasitic, adapted to life inside host’s intestines
Turbellarian reproduction Asexual - fission
Primary host (human) Intermediate host(snail) Life cycle of fluke Section 27-1 Flukes mature and reproduce sexually in the blood vessels of human intestines. Embryos are released and passed out with feces. Adult fluke Human intestine Embryo Ciliated larva Tailed larva After asexualreproduction, newlarvae are releasedfrom the snail into the water. They then infect humans, the primary host, by burrowing through the skin. If they get into the water, embryos develop into swimming larvae that infect an intermediate host (snail). Species: Schistosoma Mansoni
Brachylaimacribbi fluke • (A) adult worm • (B) two suckers and spiny coat • (C) sperm deposited from pore
Proglottid: tapeworm segment • Proglottid with fertilized eggs bursts open to release zygotes
Scolex: head with suckers or hooks Human tapeworm Parasitic tapeworm from fish
Intermediate hosts • Tapeworm larvae burrow into cow or fish muscle tissue, which infects the animal that eats this tissue. Rat tapeworm
Roundworms C. elegans
Roundworm characteristics • Unsegmented • Tapering ends • Variety of environments and climates
Roundworm characteristics • Pseudocoelomate: body cavity lined partially with mesoderm tissue • Two digestive tract openings, mouth and anus
Length of roundworm • Microscopic to 1 m in length
Feeding • Predators with grasping mouthparts • Spines to catch and eat small animals • Detritovores parasitic hookworm
Respiration, Circulation, & Excretion • Gas exchange through body walls • Metabolic waste excretion through body walls • No internal transport system
Response and movement • Simple nervous system of ganglia • Hydrostatic pressure to move around • Muscular contractions to move through water
Reproduction • Sexually • Male or female reproductive system • Internal fertilization: male deposits sperm into female • Complex life cycles in parasitic roundworms
Filarial worms • Primary cause of elephantiasis condition where parts of the body are swollen • Requires arthropod intermediate host to complete life cycle
C. elegans and research • First multicellular animal to have entire genome sequenced • Every body cell’s developmental pathway is also recorded.
Annelid characteristics • Segmented bodies • True coelom: body cavity lined with mesoderm tissue • Mouth and anus
Anus Setae Body segments Dorsalblood vessel Gizzard Crop Clitellum Mouth Brain Ganglion Circular muscle Ventralblood vessel Ringvessels Reproductiveorgans Ganglia Nephridia Longitudinalmuscle The Anatomy of an Earthworm Section 27-3 Figure 27–16
Feeding and Digestion • Filter feeders • Predators • Pharynx that may be collect or capture (if jawed) food • Food moved into esophagus, crop for storage, gizzard for grinding, digestive tract for absorption
Circulation • Closed circulatory system: blood contained within network of blood vessels • Two major blood vessels running from head to tail • Dorsal vessel: blood towards head, functions like heart • Ventral vessel: blood towards tail
Respiration • Gills: specialized for underwater gas exchange in aquatic worms (ex: tube worms) • Terrestrial annelids perform gas exchange through their moist skin.
Excretion • Nephridia: organs that filter fluid in the coelom
Response • Most have well-developed nervous systems with brain and nerve cords • Marine annelids have the best adapted nervous system.
Movement • Marine annelids may have parapodia, paddlelike appendages. • Hydrostatic skeleton: longitudinal and circular muscles • Longitudinal: contract to make worm shorter and fatter • Circular: contract to make worm longer and thinner
Reproduction • Sexual reproduction • External fertilization • May be single sex or hermaphrodite • Clitellum band thickens, secreting mucus ring with eggs where sperm are deposited • Fertilized eggs slips off with the mucus ring, which forms a cocoon where the young worms hatch later.