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Biology 2e

28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans

Biology 2e28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
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  1. Preface
  2. The Chemistry of Life
    1. 1 The Study of Life
      1. Introduction
      2. 1.1 The Science of Biology
      3. 1.2 Themes and Concepts of Biology
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 The Chemical Foundation of Life
      1. Introduction
      2. 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks
      3. 2.2 Water
      4. 2.3 Carbon
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Biological Macromolecules
      1. Introduction
      2. 3.1 Synthesis of Biological Macromolecules
      3. 3.2 Carbohydrates
      4. 3.3 Lipids
      5. 3.4 Proteins
      6. 3.5 Nucleic Acids
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  3. The Cell
    1. 4 Cell Structure
      1. Introduction
      2. 4.1 Studying Cells
      3. 4.2 Prokaryotic Cells
      4. 4.3 Eukaryotic Cells
      5. 4.4 The Endomembrane System and Proteins
      6. 4.5 The Cytoskeleton
      7. 4.6 Connections between Cells and Cellular Activities
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 5 Structure and Function of Plasma Membranes
      1. Introduction
      2. 5.1 Components and Structure
      3. 5.2 Passive Transport
      4. 5.3 Active Transport
      5. 5.4 Bulk Transport
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 6 Metabolism
      1. Introduction
      2. 6.1 Energy and Metabolism
      3. 6.2 Potential, Kinetic, Free, and Activation Energy
      4. 6.3 The Laws of Thermodynamics
      5. 6.4 ATP: Adenosine Triphosphate
      6. 6.5 Enzymes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 7 Cellular Respiration
      1. Introduction
      2. 7.1 Energy in Living Systems
      3. 7.2 Glycolysis
      4. 7.3 Oxidation of Pyruvate and the Citric Acid Cycle
      5. 7.4 Oxidative Phosphorylation
      6. 7.5 Metabolism without Oxygen
      7. 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
      8. 7.7 Regulation of Cellular Respiration
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    5. 8 Photosynthesis
      1. Introduction
      2. 8.1 Overview of Photosynthesis
      3. 8.2 The Light-Dependent Reactions of Photosynthesis
      4. 8.3 Using Light Energy to Make Organic Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    6. 9 Cell Communication
      1. Introduction
      2. 9.1 Signaling Molecules and Cellular Receptors
      3. 9.2 Propagation of the Signal
      4. 9.3 Response to the Signal
      5. 9.4 Signaling in Single-Celled Organisms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 10 Cell Reproduction
      1. Introduction
      2. 10.1 Cell Division
      3. 10.2 The Cell Cycle
      4. 10.3 Control of the Cell Cycle
      5. 10.4 Cancer and the Cell Cycle
      6. 10.5 Prokaryotic Cell Division
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  4. Genetics
    1. 11 Meiosis and Sexual Reproduction
      1. Introduction
      2. 11.1 The Process of Meiosis
      3. 11.2 Sexual Reproduction
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 12 Mendel's Experiments and Heredity
      1. Introduction
      2. 12.1 Mendel’s Experiments and the Laws of Probability
      3. 12.2 Characteristics and Traits
      4. 12.3 Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 13 Modern Understandings of Inheritance
      1. Introduction
      2. 13.1 Chromosomal Theory and Genetic Linkage
      3. 13.2 Chromosomal Basis of Inherited Disorders
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    4. 14 DNA Structure and Function
      1. Introduction
      2. 14.1 Historical Basis of Modern Understanding
      3. 14.2 DNA Structure and Sequencing
      4. 14.3 Basics of DNA Replication
      5. 14.4 DNA Replication in Prokaryotes
      6. 14.5 DNA Replication in Eukaryotes
      7. 14.6 DNA Repair
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    5. 15 Genes and Proteins
      1. Introduction
      2. 15.1 The Genetic Code
      3. 15.2 Prokaryotic Transcription
      4. 15.3 Eukaryotic Transcription
      5. 15.4 RNA Processing in Eukaryotes
      6. 15.5 Ribosomes and Protein Synthesis
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 16 Gene Expression
      1. Introduction
      2. 16.1 Regulation of Gene Expression
      3. 16.2 Prokaryotic Gene Regulation
      4. 16.3 Eukaryotic Epigenetic Gene Regulation
      5. 16.4 Eukaryotic Transcription Gene Regulation
      6. 16.5 Eukaryotic Post-transcriptional Gene Regulation
      7. 16.6 Eukaryotic Translational and Post-translational Gene Regulation
      8. 16.7 Cancer and Gene Regulation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    7. 17 Biotechnology and Genomics
      1. Introduction
      2. 17.1 Biotechnology
      3. 17.2 Mapping Genomes
      4. 17.3 Whole-Genome Sequencing
      5. 17.4 Applying Genomics
      6. 17.5 Genomics and Proteomics
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
  5. Evolutionary Processes
    1. 18 Evolution and the Origin of Species
      1. Introduction
      2. 18.1 Understanding Evolution
      3. 18.2 Formation of New Species
      4. 18.3 Reconnection and Speciation Rates
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 19 The Evolution of Populations
      1. Introduction
      2. 19.1 Population Evolution
      3. 19.2 Population Genetics
      4. 19.3 Adaptive Evolution
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 20 Phylogenies and the History of Life
      1. Introduction
      2. 20.1 Organizing Life on Earth
      3. 20.2 Determining Evolutionary Relationships
      4. 20.3 Perspectives on the Phylogenetic Tree
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  6. Biological Diversity
    1. 21 Viruses
      1. Introduction
      2. 21.1 Viral Evolution, Morphology, and Classification
      3. 21.2 Virus Infections and Hosts
      4. 21.3 Prevention and Treatment of Viral Infections
      5. 21.4 Other Acellular Entities: Prions and Viroids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 22 Prokaryotes: Bacteria and Archaea
      1. Introduction
      2. 22.1 Prokaryotic Diversity
      3. 22.2 Structure of Prokaryotes: Bacteria and Archaea
      4. 22.3 Prokaryotic Metabolism
      5. 22.4 Bacterial Diseases in Humans
      6. 22.5 Beneficial Prokaryotes
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    3. 23 Protists
      1. Introduction
      2. 23.1 Eukaryotic Origins
      3. 23.2 Characteristics of Protists
      4. 23.3 Groups of Protists
      5. 23.4 Ecology of Protists
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 24 Fungi
      1. Introduction
      2. 24.1 Characteristics of Fungi
      3. 24.2 Classifications of Fungi
      4. 24.3 Ecology of Fungi
      5. 24.4 Fungal Parasites and Pathogens
      6. 24.5 Importance of Fungi in Human Life
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 25 Seedless Plants
      1. Introduction
      2. 25.1 Early Plant Life
      3. 25.2 Green Algae: Precursors of Land Plants
      4. 25.3 Bryophytes
      5. 25.4 Seedless Vascular Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    6. 26 Seed Plants
      1. Introduction
      2. 26.1 Evolution of Seed Plants
      3. 26.2 Gymnosperms
      4. 26.3 Angiosperms
      5. 26.4 The Role of Seed Plants
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 27 Introduction to Animal Diversity
      1. Introduction
      2. 27.1 Features of the Animal Kingdom
      3. 27.2 Features Used to Classify Animals
      4. 27.3 Animal Phylogeny
      5. 27.4 The Evolutionary History of the Animal Kingdom
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 28 Invertebrates
      1. Introduction
      2. 28.1 Phylum Porifera
      3. 28.2 Phylum Cnidaria
      4. 28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
      5. 28.4 Superphylum Lophotrochozoa: Molluscs and Annelids
      6. 28.5 Superphylum Ecdysozoa: Nematodes and Tardigrades
      7. 28.6 Superphylum Ecdysozoa: Arthropods
      8. 28.7 Superphylum Deuterostomia
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    9. 29 Vertebrates
      1. Introduction
      2. 29.1 Chordates
      3. 29.2 Fishes
      4. 29.3 Amphibians
      5. 29.4 Reptiles
      6. 29.5 Birds
      7. 29.6 Mammals
      8. 29.7 The Evolution of Primates
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  7. Plant Structure and Function
    1. 30 Plant Form and Physiology
      1. Introduction
      2. 30.1 The Plant Body
      3. 30.2 Stems
      4. 30.3 Roots
      5. 30.4 Leaves
      6. 30.5 Transport of Water and Solutes in Plants
      7. 30.6 Plant Sensory Systems and Responses
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 31 Soil and Plant Nutrition
      1. Introduction
      2. 31.1 Nutritional Requirements of Plants
      3. 31.2 The Soil
      4. 31.3 Nutritional Adaptations of Plants
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 32 Plant Reproduction
      1. Introduction
      2. 32.1 Reproductive Development and Structure
      3. 32.2 Pollination and Fertilization
      4. 32.3 Asexual Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. Animal Structure and Function
    1. 33 The Animal Body: Basic Form and Function
      1. Introduction
      2. 33.1 Animal Form and Function
      3. 33.2 Animal Primary Tissues
      4. 33.3 Homeostasis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 34 Animal Nutrition and the Digestive System
      1. Introduction
      2. 34.1 Digestive Systems
      3. 34.2 Nutrition and Energy Production
      4. 34.3 Digestive System Processes
      5. 34.4 Digestive System Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 35 The Nervous System
      1. Introduction
      2. 35.1 Neurons and Glial Cells
      3. 35.2 How Neurons Communicate
      4. 35.3 The Central Nervous System
      5. 35.4 The Peripheral Nervous System
      6. 35.5 Nervous System Disorders
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 36 Sensory Systems
      1. Introduction
      2. 36.1 Sensory Processes
      3. 36.2 Somatosensation
      4. 36.3 Taste and Smell
      5. 36.4 Hearing and Vestibular Sensation
      6. 36.5 Vision
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 37 The Endocrine System
      1. Introduction
      2. 37.1 Types of Hormones
      3. 37.2 How Hormones Work
      4. 37.3 Regulation of Body Processes
      5. 37.4 Regulation of Hormone Production
      6. 37.5 Endocrine Glands
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 38 The Musculoskeletal System
      1. Introduction
      2. 38.1 Types of Skeletal Systems
      3. 38.2 Bone
      4. 38.3 Joints and Skeletal Movement
      5. 38.4 Muscle Contraction and Locomotion
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    7. 39 The Respiratory System
      1. Introduction
      2. 39.1 Systems of Gas Exchange
      3. 39.2 Gas Exchange across Respiratory Surfaces
      4. 39.3 Breathing
      5. 39.4 Transport of Gases in Human Bodily Fluids
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    8. 40 The Circulatory System
      1. Introduction
      2. 40.1 Overview of the Circulatory System
      3. 40.2 Components of the Blood
      4. 40.3 Mammalian Heart and Blood Vessels
      5. 40.4 Blood Flow and Blood Pressure Regulation
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    9. 41 Osmotic Regulation and Excretion
      1. Introduction
      2. 41.1 Osmoregulation and Osmotic Balance
      3. 41.2 The Kidneys and Osmoregulatory Organs
      4. 41.3 Excretion Systems
      5. 41.4 Nitrogenous Wastes
      6. 41.5 Hormonal Control of Osmoregulatory Functions
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    10. 42 The Immune System
      1. Introduction
      2. 42.1 Innate Immune Response
      3. 42.2 Adaptive Immune Response
      4. 42.3 Antibodies
      5. 42.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    11. 43 Animal Reproduction and Development
      1. Introduction
      2. 43.1 Reproduction Methods
      3. 43.2 Fertilization
      4. 43.3 Human Reproductive Anatomy and Gametogenesis
      5. 43.4 Hormonal Control of Human Reproduction
      6. 43.5 Human Pregnancy and Birth
      7. 43.6 Fertilization and Early Embryonic Development
      8. 43.7 Organogenesis and Vertebrate Formation
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
  9. Ecology
    1. 44 Ecology and the Biosphere
      1. Introduction
      2. 44.1 The Scope of Ecology
      3. 44.2 Biogeography
      4. 44.3 Terrestrial Biomes
      5. 44.4 Aquatic Biomes
      6. 44.5 Climate and the Effects of Global Climate Change
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 45 Population and Community Ecology
      1. Introduction
      2. 45.1 Population Demography
      3. 45.2 Life Histories and Natural Selection
      4. 45.3 Environmental Limits to Population Growth
      5. 45.4 Population Dynamics and Regulation
      6. 45.5 Human Population Growth
      7. 45.6 Community Ecology
      8. 45.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior
      9. Key Terms
      10. Chapter Summary
      11. Visual Connection Questions
      12. Review Questions
      13. Critical Thinking Questions
    3. 46 Ecosystems
      1. Introduction
      2. 46.1 Ecology of Ecosystems
      3. 46.2 Energy Flow through Ecosystems
      4. 46.3 Biogeochemical Cycles
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    4. 47 Conservation Biology and Biodiversity
      1. Introduction
      2. 47.1 The Biodiversity Crisis
      3. 47.2 The Importance of Biodiversity to Human Life
      4. 47.3 Threats to Biodiversity
      5. 47.4 Preserving Biodiversity
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
  10. A | The Periodic Table of Elements
  11. B | Geological Time
  12. C | Measurements and the Metric System
  13. Index
By the end of this section, you will be able to do the following:
  • Describe the unique anatomical and morphological features of flatworms, rotifers, and Nemertea
  • Identify an important extracoelomic cavity found in Nemertea
  • Explain the key features of Platyhelminthes and their importance as parasites

Animals belonging to superphylum Lophotrochozoa are triploblastic (have three germ layers) and unlike the cnidarians, they possess an embryonic mesoderm sandwiched between the ectoderm and endoderm. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are superficially symmetrical. In these phyla, we also see the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism—exactly where a mobile bilaterally symmetrical organism first encounters its environment.

Lophotrochozoa are also protostomes, in which the blastopore, or the point of invagination of the ectoderm (outer germ layer), becomes the mouth opening into the alimentary canal. This developmental pattern is called protostomy or “first mouth.” Protostomes include acoelomate, pseudocoelomate, and eucoelomate phyla. The coelom is a cavity that separates the ectoderm from the endoderm. In acoelomates, a solid mass of mesoderm is sandwiched between the ectoderm and endoderm and does not form a cavity or “coelom,” leaving little room for organ development; in pseudocoelomates, there is a cavity or pseudocoelom that replaces the blastocoel (the cavity within the blastula), but it is only lined by mesoderm on the outside of the cavity, leaving the gut tube and organs unlined; in eucoelomates, the cavity that obliterates the blastocoel as the coelom develops is lined both on the outside of the cavity (parietal layer) and also on the inside of the cavity, around the gut tube and the internal organs (visceral layer).

Eucoelmate protostomes are schizocoels, in which mesoderm-producing cells typically migrate into the blastocoel during gastrulation and multiply to form a solid mass of cells. Cavities then develop within the cell mass to form the coelom. Since the forming body cavity splits the mesoderm, this protostomic coelom is termed a schizocoelom. As we will see later in this chapter, chordates, the phylum to which we belong, generally develop a coelom by enterocoely: pouches of mesoderm pinch off the invaginating primitive gut, or archenteron, and then fuse to form a complete coelom. We should note here that a eucoelomate can form its “true coelom” by either schizocoely or enterocoely. The process that produces the coelom is different and of taxonomic importance, but the result is the same: a complete, mesodermally lined coelom.

Most organisms placed in the superphylum Lophotrochozoa possess either a lophophore feeding apparatus or a trochophore larvae (thus the contracted name, “lopho-trocho-zoa”). The lophophore is a feeding structure composed of a set of ciliated tentacles surrounding the mouth. A trochophore is a free-swimming larva characterized by two bands of cilia surrounding a top-like body. Some of the phyla classified as Lophotrochozoa may be missing one or both of these defining structures. Nevertheless their placement with the Lophotrochozoa is upheld when ribosomal RNA and other gene sequences are compared. The systematics of this complex group is still unclear and much more work remains to resolve the cladistic relationships among them.

Phylum Platyhelminthes

The flatworms are acoelomate organisms that include many free-living and parasitic forms. The flatworms possess neither a lophophore nor trochophore larvae, although the larvae of one group of flatworms, the Polycladida (named after its many-branched digestive tract), are considered to be homologous to trochophore larvae. Spiral cleavage is also seen in the polycladids and other basal flatworm groups. The developmental pattern of some of the free-living forms is obscured by a phenomenon called "blastomere anarchy," in which a sort of temporary feeding larva forms, followed by a regrouping of cells within the embryo that gives rise to a second-stage embryo. However, both the monophyly of the flatworms and their placement in the Lophotrochozoa has been supported by molecular analyses.

The Platyhelminthes consist of two monophyletic lineages: the Catenulida and the Rhabditophora. The Catenulida, or "chain worms," is a small clade of just over 100 species. These worms typically reproduce asexually by budding. However, the offspring do not fully detach from the parents and the formation resembles a chain in appearance. All of the flatworms discussed here are part of the Rhabditophora ("rhabdite bearers"). Rhabdites are rodlike structures discharged in the mucus produced by some free-living flatworms; Eucoelmate protostomes are schizocoels, in which mesoderm-producing cells typically migrate into the blastocoel during gastrulation likely serve in both defense and to provide traction for ciliary gliding along the substrate. Unlike free-living flatworms, many species of trematodes and cestodes are parasitic, including important parasites of humans.

This image shows two flatworms with significant physical diversity.  One flatworm is brightly colored, and is long and tubular.  The other flatworm is broad, and flat, and a dark color.
Figure 28.13 Flatworms exhibit significant diversity. (a) A blue Pseudoceros flatworm (Pseudoceros bifurcus); (b) gold speckled flatworm (Thysanozoon nigropapillosum). (credit a: modification of work by Stephen Childs; b: modification of work by Pril Fish.)

Flatworms have three embryonic tissue layers that give rise to epidermal tissues (from ectoderm), the lining of the digestive system (from endoderm), and other internal tissues (from mesoderm). The epidermal tissue is a single layer of cells or a layer of fused cells (syncytium) that covers two layers of muscle, one circular and the other longitudinal. The mesodermal tissues include mesenchymal cells that contain collagen and support secretory cells that produce mucus and other materials at the surface. Because flatworms are acoelomates, the mesodermal layer forms a solid mass between the outer epidermal surface and the cavity of the digestive system.

Physiological Processes of Flatworms

The free-living species of flatworms are predators or scavengers. Parasitic forms feed by absorbing nutrients provided by their hosts. Most flatworms, such as the planarian shown in Figure 28.14, have a branching gastrovascular cavity rather than a complete digestive system. In such animals, the “mouth” is also used to expel waste materials from the digestive system, and thus also serves as an anus. (A few species may have a second anal pore or opening.) The gut may be a simple sac or highly branched. Digestion is primarily extracellular, with digested materials taken into the cells of the gut lining by phagocytosis. One parasitic group, the tapeworms (cestodes), lacks a digestive system altogether, and absorb digested food from the host.

Flatworms have an excretory system with a network of tubules attached to flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body through excretory pores. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of lateral nerve cords running the length of the body with transverse connections between them. Two large cerebral ganglia—concentrations of nerve cell bodies at the anterior end of the worm—are associated with photosensory and chemosensory cells. There is neither a circulatory nor a respiratory system, with gas and nutrient exchange dependent on diffusion and cell-to-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (both male and female reproductive organs are found in the same individual), and fertilization is typically internal. Asexual reproduction by fission is common in some groups.

Illustration shows the digestive, nervous and excretory systems in a flat, worm-like planaria. The digestive system starts at the ventral mouth opening in the middle of the animal, and then extends to the head through the middle of the body, and toward the along the sides of the body. Many lateral branches occur along the digestive system. The nervous system has 2 cerebral ganglia at the eyes in the head, and 2 ventral nerve cords with transverse connections along the length of the body to the tail. The excretory system is arranged in 2 long mesh-like structures down each side of the body.
Figure 28.14 Planaria, a free-living flatworm. The planarian is a flatworm that has a gastrovascular cavity with one opening that serves as both mouth and anus. The excretory system is made up of flame cells and tubules connected to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords running the length of the body, with cerebral ganglia and eyespots at the anterior end.

Diversity of Flatworms

The flatworms have been traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 28.15). However, the relationships among members of these classes has recently been reassessed, with the turbellarians in particular now viewed as paraphyletic, since its descendants may also include members of the other three classes. Members of the clade or class Rhabditophora are now dispersed among multiple orders of Platyhelminthes, the most familiar of these being the Polycladida, which contains the large marine flatworms; the Tricladida (which includes Dugesia [“planaria”] and Planaria and its relatives); and the major parasitic orders: Monogenea (fish ectoparasites), Trematoda (flukes), and Cestoda (tapeworms), which together form a monophyletic clade.

Photo A shows a Bedford's flatworm from the class Turbellaria. The worm has the appearance of a fringed ribbon, black with pink stripes, swimming above the sand. Photo B shows a Dactylogyrus from the class Monogenea. The worm's body is a long, thin translucent oval with bulges at one end that give the appearance of a head. Three dark spots appear in the head, and four more dark spots three-quarters appear of the way down the body. Anchors that enable the worm to latch onto gills are located near these spots. Photo C shows a foot- shaped brown worm. Photo D shows a long, thin ribbon-like white worm.
Figure 28.15 Traditional flatworm classes. Phylum Platyhelminthes was previously divided into four classes. (a) Class Turbellaria includes the free-living polycladid Bedford’s flatworm (Pseudobiceros bedfordi), which is about 8 to 10 cm in length. (b) The parasitic class Monogenea includes Dactylogyrus spp, commonly called gill flukes, which are about 0.2 mm in length and have two anchors, indicated by arrows used to attach the parasite on to the gills of host fish. (c) The class Trematoda includes Fascioloides magna (right) and Fasciola hepatica (two specimens on left, also known as the common liver fluke). (d) Class Cestoda includes tapeworms such as this Taenia saginata, infects both cattle and humans, and can reach 4 to 10 meters in length; the specimen shown here is about four meters long. (credit a: modification of work by Jan Derk; credit d: modification of work by CDC)

Most free-living flatworms are marine polycladids, although tricladid species live in freshwater or moist terrestrial environments, and there are a number of members from other orders in both environments. The ventral epidermis of free-living flatworms is ciliated, which facilitates their locomotion. Some free-living flatworms are capable of remarkable feats of regeneration in which an individual may regrow its head or tail after being severed, or even several heads if the planaria is cut lengthwise.

The monogeneans are ectoparasites, mostly of fish, with simple life cycles that consist of a free-swimming larva that attaches to a fish, prior to its transformation to the ectoparasitic adult form. The parasite has only one host and that host is usually very specific. The worms may produce enzymes that digest the host tissues, or they may simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs. The primary host is usually a vertebrate and the secondary host is almost always a mollusk, in which multiple larvae are produced asexually. Trematodes, which attached internally to the host via an oral and medial sucker, are responsible for serious human diseases including schistosomiasis, caused by several species of the blood fluke, Schistosoma spp. Various forms of schistosomiasis infect an estimated 200 million people in the tropics, leading to organ damage, secondary infection by bacteria, and chronic symptoms like fatigue. Infection occurs when the human enters the water and metacercaria larvae, released from the snail host, locate and penetrate the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing.

Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the snail host. The eggs, which have a barb on them, can damage the vascular system of the human host, causing ulceration, abscesses, and bloody diarrhea, wherever they reside, thereby allowing other pathogens to cause secondary infections. In fact, it is the parasite’s eggs that produce most of the main ill effects of schistosomiasis. Many eggs do not make the transit through the veins of the host for elimination, and are swept by blood flow back to the liver and other locations, where they can cause severe inflammation. In the liver, the errant eggs may impede circulation and cause cirrhosis. Control is difficult in impoverished areas in unsanitary, crowded conditions, and prognosis is poor in people with heavy infections of Schistosoma japonicum, without early treatment.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 28.16). Tapeworms, such as those of Taenia spp, live in the intestinal tract of the primary host and remain fixed using a sucker or hooks on the anterior end, or scolex, of the tapeworm body, which is essentially a colony of similar subunits called proglottids. Each proglottid may contain an excretory system with flame cells, along with reproductive structures, both male and female. Because they are so long and flat, tapeworms do not need a digestive system; instead, they absorb nutrients from the food matter surrounding them in the host’s intestine by diffusion.

Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization between different worms in the same host, but may also occur between proglottids. The mature proglottids detach from the body of the worm and are released into the feces of the organism. The eggs are eaten by an intermediate host, typically another vertebrate. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. When the muscle tissue is consumed by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked or poorly cooked pork, beef, or fish.

The tapeworm life cycle begins when eggs or tapeworm segments, called proglottids, pass from human feces into the environment. Taenia saginata infects cattle and Taenia solium infects pigs when they eat contaminated vegetation. The embryo penetrates the animal intestinal wall and takes up residence in muscle tissue, where it transforms into the larval form. Humans who consume raw or undercooked infected meat become infected when the tapeworm attaches itself to the intestinal wall via suckers or hooks on the scolex, or head. The mature worm produces proglottids and eggs, which pass from the body in feces, completing the cycle.
Figure 28.16 Tapeworm life cycle. Tapeworm (Taenia spp.) infections occur when humans consume raw or undercooked infected meat. (credit: modification of work by CDC)

Phylum Rotifera

The rotifers ("wheel-bearer") belong to a group of microscopic (about 100 µm to 2 mm) mostly aquatic animals that get their name from the corona—a pair of ciliated feeding structures that appear to rotate when viewed under the light microscope (Figure 28.17). Although their taxonomic status is currently in flux, one treatment places the rotifers in three classes: Bdelloidea, Monogononta, and Seisonidea. In addition, the parasitic “spiny headed worms” currently in phylum Acanthocephala, appear to be modified rotifers and will probably be placed into the group in the near future. Undoubtedly the rotifers will continue to be revised as more phylogenetic evidence becomes available.

The pseudocoelomate body of a rotifer is remarkably complex for such a small animal (roughly the size of a Paramecium) and is divided into three sections: a head (which contains the corona), a trunk (which contains most of the internal organs), and the foot. A cuticle, rigid in some species and flexible in others, covers the body surface. They have both skeletal muscle associated with locomotion and visceral muscles associated with the gut, both composed of single cells. Rotifers are typically free-swimming or planktonic (drifting) organisms, but the toes or extensions of the foot can secrete a sticky material to help them adhere to surfaces. The head contains a number of eyespots and a bilobed “brain,” with nerves extending into the body.

Scanning electron micrograph A shows rotifers from the class Bdelloidea, which have a long, tube-shaped body with a fringe surrounding the mouth. Light micrograph B shows that Polyarthra from the class Monogononta is shorter and wider than the bdelloid rotifers, with a smaller fringe.
Figure 28.17 Rotifers. Shown are examples from two of the three classes of rotifer. (a) Species from the class Bdelloidea are characterized by a large corona. The whole animals in the center of this scanning electron micrograph are shown surrounded by several sets of jaws from the mastax of rotifers. (b) Polyarthra, from the largest rotifer class Monogononta, has a smaller corona than bdelloid rotifers, and a single gonad, which give the class its name. (credit a: modification of work by Diego Fontaneto; credit b: modification of work by U.S. EPA; scale-bar data from Cory Zanker)

Rotifers are commonly found in freshwater and some saltwater environments throughout the world. As filter feeders, they will eat dead material, algae, and other microscopic living organisms, and are therefore very important components of aquatic food webs. A rotifer's food is directed toward the mouth by the current created from the movement of the coronal cilia. The food particles enter the mouth and travel first to the mastax—a muscular pharynx with toothy jaw-like structures. Examples of the jaws of various rotifers are seen in Figure 28.17a. Masticated food passes near digestive and salivary glands, into the stomach, and then to the intestines. Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus.

Link to Learning

Watch this video to see rotifers feeding.

About 2,200 species of rotifers have been identified. Figure 28.18 shows the anatomy of a rotifer belonging to class Bdelloidea. Some rotifers are dioecious organisms and exhibit sexual dimorphism (males and females have different forms). In many dioecious species, males are short-lived and smaller with no digestive system and a single testis. Many rotifer species exhibit haplodiploidy, a method of sex determination in which a fertilized egg develops into a female and an unfertilized egg develops into a male. However, reproduction in the bdelloid rotifers is exclusively parthenogenetic and appears to have been so for millions of years: Thus, all bdelloid rotifers and their progeny are female! The bdelloids may compensate for this genetic insularity by borrowing genes from the DNA of other species. Up to 10% of a bdelloid genome comprises genes imported from related species. Some rotifer eggs are capable of extended dormancy for protection during harsh environmental conditions.

The illustration shows long, tube-shaped animal with a crown-like corona on top. Cilia fringe the top of the corona. Between the two lobes of the corona is the mouth, which leads to the stomach, intestine, and anus. The mastax surround the mouth, and beneath the mastax is a digestive gland. The pseudocoel surrounds the stomach. At the bottom if the animal is a foot that stands on two toes.
Figure 28.18 A bdelloid rotifer. This illustration shows the anatomy of a bdelloid rotifer.

Phylum Nemertea

The Nemertea are colloquially known as ribbon worms or proboscis worms. Most species of phylum Nemertea are marine and predominantly benthic (bottom dwellers), with an estimated 900 known species. However, nemerteans have been recorded in freshwater and very damp terrestrial habitats as well. Most nemerteans are carnivores, feeding on worms, clams, and crustaceans. Some species are scavengers, and some, like Malacobdella grossa, have also evolved commensal relationships with mollusks. Economically important species have at times devastated commercial fishing of clams and crabs. Nemerteans have almost no predators and two species are sold as fish bait.

Morphology

Nemerteans vary in size from 1 cm to several meters. They show bilateral symmetry and remarkable contractile properties. Because of their contractility, they can change their morphological presentation in response to environmental cues. Animals in phylum Nemertea are soft and unsegmented animals, with a morphology like a flattened tube. (Figure 28.19).

The photo shows a worm that resembles intestines, sitting on the muddy ocean floor.  The worm is a long tubular creature whose body overlaps itself, and its surface appears to be twisted at parts.
Figure 28.19 A proboscis worm. The proboscis worm (Parborlasia corrugatus) is a scavenger that combs the sea floor for food. The species is a member of the phylum Nemertea. The specimen shown here was photographed in the Ross Sea, Antarctica. (credit: Henry Kaiser, National Science Foundation)

A unique characteristic of this phylum is the presence of an eversible proboscis enclosed in a pocket called a rhynchocoel (not part of the animal's actual coelom). The proboscis is located dorsal to the gut and serves as a harpoon or tentacle for food capture. In some species it is ornamented with barbs. The rhynchocoel is a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals (Figure 28.20). The proboscis may be extended by hydrostatic pressure generated by contraction of muscle of the rhynchocoel and retracted by a retractor muscle attached to the rear wall of the rhynchocoel.

The illustration shows worm-shaped animal with fringe-like sensory papillae at one end. The mouth, which is part way down the body, leads to a stomach and intestine, then empties into an anus at the far end. The cerebral ganglia are located above the mouth. Lateral nerve cords run down either side of the animal from the central ganglia. The proboscis is a long, thin structure inside a cavity called the rhynchocoel.
Figure 28.20 The anatomy of a Nemertean is shown.

Link to Learning

Watch this video to see a nemertean attack a polychaete with its proboscis.

Digestive System

The nemerteans, which are primarily predators of annelids and crustaceans, have a well-developed digestive system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. The intestine is present in the form of diverticular pouches and ends in a rectum that opens via an anus. Gonads are interspersed with the intestinal diverticular pouches and open outward via genital pores.

Nemerteans are sometimes classified as acoels, but because their closed circulatory system is derived from the coelomic cavity of the embryo, they may be regarded as coelomic. Their circulatory system consists of a closed loop formed by a connected pair of lateral blood vessels. Some species may also have a dorsal vessel or cross-connecting vessels in addition to lateral ones. Although the circulatory fluid contains cells, it is often colorless. However, the blood cells of some species bear hemoglobin as well as other yellow or green pigments. The blood vessels are contractile, although there is usually no regular circulatory pathway, and movement of blood is also facilitated by the contraction of muscles in the body wall. The circulation of fluids in the rhychocoel is more or less independent of the blood circulation, although blind branches from the blood vessels into the rhyncocoel wall can mediate exchange of materials between them. A pair of protonephridia, or excretory tubules, is present in these animals to facilitate osmoregulation. Gaseous exchange occurs through the skin.

Nervous System

Nemerteans have a "brain" composed of four ganglia situated at the anterior end, around the rhynchocoel. Paired longitudinal nerve cords emerge from the brain ganglia and extend to the posterior end. Additional nerve cords are found in some species. Interestingly, the brain can contain hemoglobin, which acts as an oxygen reserve. Ocelli or eyespots are present in pairs, in multiples of two in the anterior portion of the body. It is speculated that the eyespots originate from neural tissue and not from the epidermis.

Reproduction

Nemerteans, like flatworms, have excellent powers of regeneration, and asexual reproduction by fragmentation is seen in some species. Most animals in phylum Nemertea are dioecious, although freshwater species may be hermaphroditic. Stem cells that become gametes aggregate within gonads placed along the digestive tract. Eggs and sperm are released into the water, and fertilization occurs externally. Like most lophotrochozoan protostomes, cleavage is spiral, and development is usually direct, although some species have a trochophore-like larva, in which a young worm is constructed from a series of imaginal discs that begin as invaginations from the body surface of the larva.

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