Flatworms, Mesozoans, and Ribbon Worms

1. Flatworms, Mesozoans, and Ribbon Worms Chapter 14

2. Bilateria • Most animals have bilateral symmetry. • The vast majority of animal species belong to the clade Bilateria, which consists of animals with bilateral symmetry and triploblastic development.

3. Bilateral Symmetry • Radially symmetrical animals have the world coming at them from all directions. • They can catch prey coming from any direction. • Animals that begin to move about actively seeking food need a different body organization. • Distinct head end with sensory structures. • Cephalization

4. Bilateral Symmetry • Animals with bilateral symmetry have a distinct head end and can be divided into right and left halves.

5. Acoelomate Bilateral Animals • Animals that have no space between their gut and body wall are said to be acoelomate. • These animals are also triploblastic – they have three embryonic germ layers. • Organ-system level of organization – more division of labor among their organs.

6. Acoelomates • Although flatworms undergo triploblastic development, they are acoelomates.

7. Acoelomates • These acoelomate phyla are protostomes and have spiral cleavage. • Most have determinate cleavage. • These are the simplest animals with an excretory system. • Acoelomate phyla belong to the superphylum Lophotrochozoa

8. Phylum Acoelomorpha • Group contains ~350 species. • Members were formerly in Class Turbellaria within phylum Platyhelminthes Small flat worms less than 5 mm in length. • Typically live in marine sediments; few are pelagic. • Some species live in brackish water. • Most symbiotic but some parasitic. • Have a cellular ciliated epidermis. • Parenchyma layer contains small amount of ECM and circular, longitudinal, and diagonal muscles.

9. Phylum Acoelomorpha - Digestion and Nutrition • Incomplete digestive system - no anus. • In many acoels, the gut and pharynx are absent. • Phagocytotic cells digest food intracellularly when food is passed into temporary spaces.

10. Phylum Acoelomorpha - Reproduction • Monoecious • Female produces yolk-filled, endolecithal eggs. • Following fertilization some or all cleavage events produce a duet-spiral pattern of new cells. • May be a defining character for acoelomorphs.

11. Phylum Platyhelminthes • Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats.

12. Phylum Platyhelminthes • Flatworms are flattened dorsoventrally and have a gastrovascular cavity. • Extracellular digestion. • Undigested food is egested through the pharynx.

13. Phylum Platyhelminthes • The osmoregulatory system consists of protonephridia (excretory or osmoregulatory organs closed at the inner end) with flame cells. • Most metabolic wastes removed by diffusion across the body wall.

14. Phylum Platyhelminthes • The nervous system consists of a ladder-like network of nerves and a bilobed brain. • Many have large ocelli – light sensing organs.

15. Phylum Platyhelminthes • Many can reproduce asexually as well as sexually. • Asexual reproduction via fission. • Sometimes the new individuals remain attached – chains of zooids. • Monoecious

16. Taxonomy • Flatworms (phylum Platyhelminthes) are divided into four classes: • Class Turbellaria – ex. Planaria • Not monophyletic • Class Trematoda – parasitic flukes • Class Monogenea – parasitic monogenetic flukes • Class Cestoda - tapeworms

17. Phylum Platyhelminthes

18. Class Turbellaria • Turbellarians are nearly all free-living and mostly marine.

19. Class Turbellaria • The best-known turbellarians, commonly called planarians, have light-sensitive eyespots and centralized nerve nets.

20. Class Trematoda • Trematodes live as parasites in or on other animals. • They parasitize a wide range of hosts.

21. Class Trematoda • Subclass Digenea, digenetic flukes, have a complex life cycle with a mollusc (snail) as the first host and a vertebrate as the final, or definitive, host.

22. Class Monogenea • All monogeneans are parasites. • Often found in the gills or external surfaces of fishes.

23. Class Cestoda • Tapeworms (Class Cestoda) are also parasitic and lack a digestive system. • The scolex is equipped with suckers and hooks for attachment to the host. • Each proglottid contains a set of reproductive organs.

24. Class Cestoda • Cestodes usually require at least two hosts. • Adult cestodes are parasites in the digestive tracts of vertebrates.

25. Phylum Mesozoa • Phylum Mesozoa is considered a “missing link” between protozoa and metazoa. • Have a simple level of organization. • Minute, ciliated, and wormlike animals. • All live as parasites in marine invertebrates. • Most composed of only 20 to 30 cells arranged in two layers. • Layers are not homologous to germ layers of other metazoans. • Two classes, Rhombozoa and Orthonectida, are so different that some authorities place them in separate phyla.

26. Phylum Mesozoa • Rhombozoans live in kidneys of benthic cephalopods. • Adults called vermiforms and are long and slender. • Inner, reproductive cells give rise to vermiform larvae. • When overpopulated, reproductive cells develop into gonad-like structures producing male and female gametes. • Larvae are shed with host urine into the seawater.

27. Phylum Mesozoa • Orthonectids parasitize a variety of invertebrates. • Reproduce sexually and asexually. • Asexual reproduction consists of a multinucleated mass called a plasmodium.

28. Phylogeny of Mesozoans • Some consider these organisms primitive flatworms and place them in phylum Platyhelminthes. • Molecular evidence groups them with flatworms in superphylum Lophotrochozoa. • However, molecular phylogeny that included an orthonectid and two species from a rhombozoan subgroup, the dicyemids, did not show members of the two classes to be sister taxa. • The phylum may not be monophyletic.

29. Phylum Nemertea • Ribbon worms, phylum Nemertea, use a proboscis to capture prey. • Almost completely marine. • Active predators. • General body plan similar to turbellarians.

30. Phylum Nemertea • An anus is present providing these worms with a complete digestive system. • Nermeteans are the simplest animals to have a closed loop blood-vascular system.

31. Phylogeny • A planuloid ancestor (like the planula larva of cnidarians?) may have given rise to a branch of descendents that were sessile or free floating and radial Cnidaria. • Another branch acquired a creeping habit and bilateral symmetry Bilateria.