Bilateria

1. Bilateria • Having completed the radially symmetrical Cnidarians and Ctenophores we now move on to the remaining animals, all of which are bilaterally symmetrical (or secondarily radially symmetrical [the Echinoderms]). • These are the Bilateria.

2. Bilateria • All Bilateria are triploblastic and the members are assigned to two major groups, which we have already met, the Protostomia and the Deuterostomia. • Deuterostomes include the Echindoerms, Hemichordates and Chordates. • Protostomes include all the other bilateral invertebrates including Platyhelminthes, Annelida, Mollusca, and Arthropoda.

5. Protostomia divisions • Classification of invertebrates is in a state of flux for several reasons: • molecular phylogenetic studies have cast doubt on traditional invertebrate classification. • many invertebrates are soft-bodied so fossils of many groups are rare or unknown, which makes relationships between groups hard to decipher. • Traditional major groups of protostomes (acoelomate, pseudocoelomate and coelomate) appear not to be monophyletic. • For this class, we will use the Deuterostome/Protostome arrangement and recognize two majors groups of Protostomes: the Lophotrochozoa and the Ecdysozoa.

6. Protostome Divisions: the Lophotrochozoa and Ecdysozoa. • Protostomes are divided into two large groups the Lophotrochozoa and the Ecdysozoa. • The relatedness of phyla within these two groups is not entirely clear and will likely change in the future.

7. Lophotrochozoa and Ecdysozoa • Lophotrochozoa: members generally possess a trochophore larva [free-swimming oval or pyramidal ciliated larva with a band of cilia around the body] or a lophophore [tentacle bearing arm which contains within it an extension of the coelomic cavity]. • Ecdysozoa: members shed their cuticle as they grow

8. Trochophore larva http://www.microscopy-uk.org.uk/mag/imgjan09/image006.jpg

9. Bryozoan lophophore http://www.bryozoans.nl/pictures/ figuren/anatomy.jpg Lophophore: characteristic feeding structure of members of the Brachiopoda, Bryozoa and Phoronida

10. Lophotrochozoa and Ecdysozoa • Lophotrochozoa members: • Platyhelminthes, Annelida, Mollusca, and a diverse array of “lesser phyla” including Nemertea, Gnathostomulida, Rotifera, and Sipuncula. • Ecdysozoa members: • Nematoda, Arthropoda and “lesser phyla” including Onychophora, Tardigrada, and Priapulida.

11. “Worms” • The term worm is loosely employed in biology and is applied to very different animals including the segmented worms (Annelids), roundworms (pseudocoelomates) and a variety of acoelomate bilateral animals. • “Worm” describes any bilaterally symmetrical, legless, soft-bodied animal at least 2-3 times as long as it is wide. • One group of worms traditionally recognized was the acoelomate worms.

12. Acoelomate worms • Three phyla of acoelomate worms were traditionally grouped together because they lacked a coelom and had a solid body filled with parenchyma cells: • Platyhelminthes: flatworms • Nemertea: ribbon worms • Gnathostomulida: jawed worms

13. Baseodiscus delineatus (Nemertea) http://tolweb.org/images/Nemertea/2489

14. http://tolweb.org/images/Gnathostomulida/2481

15. Acoelomate worms • However, molecular evidence based on ribosomal RNA sequences suggests the Nemertea are more closely related to Annelids than Platyhelminths. • Similarly, Gnathostomulids do not have free swimming larvae and their sperm morphology is very different from the Platyhelminthes, which suggests they aren’t close relatives.

16. Acoelomate worms • Because of the dubious relatedness of the phylum Platyhelminthes to these other groups we will not deal with them until later in the semester. • However, the phylum Platyhelminthes, is a very important group which includes a wide variety of parasitic forms such as the flukes and tapeworms.

17. Phylum Platyhelminthes • Unlike other animals encountered so far, Platyhelminthes: • have evolved cephalization with their sense organs concentrated at the head end. • Possess the beginning of a ladder-type nervous system. • are bilaterally symmetrical. • Are dorsoventrally flattened

18. Phylum Platyhelminthes • In addition they are • triploblastic, but lack a coelom. Instead, they have a solid body filled with parenchyma cells. • have evolved organs and in some cases organ systems. • The first (and simplest) excretory or osmoregulatory systems and circulatory systems are found in members of these groups.

20. Phylum Platyhelminthes • Platyhelminthes typically have dorsoventrally flattened bodies, usually slender and leaflike or ribbonlike. • The advantage of a flat body is that it increases surface area and allows the animal to exchange gas and lose wastes by diffusion. • Four classes in the Platyhelminthes. The Turbellaria are free living whereas as members of the Monogenea, Trematoda and Cestoda are parasitic.

21. Nutrition • The digestive system includes a mouth, pharynx, and blind intestine (the gut is incomplete) • In the free-living Turbellarians the pharynx can be everted from the mouth. • Food is sucked into the intestine where a combination of extracellular and intracellular digestion takes place.

22. http://www.thaigoodview.com/library/contest2551/science04/119http://www.thaigoodview.com/library/contest2551/science04/119 /kingdon_animalia/images/turbellaria4.jpg

23. “Organization of the blind digestive cavity of polyclads [a group of Turbellarians] with highly branched diverticles (ventral view)” http://www.rzuser.uni-heidelberg.de/~bu6/Introduction04.html

24. Nutrition • Undigested food exits via the pharynx. • In the Cestoda the digestive tract is absent and all nutrients are absorbed across the tegument (the syncytial membrane/body covering found in all parasitic Platyhelminthes).

25. Excretion/Osmoregulation • The osmoregulatory system consists of a series of canals that end in flame cells or protonephridia. • The flame cell consists of a fine-meshed cup that contains cilia. The beating of the cilia draws fluid which is filtered as it passes into the cup. • This system appears mainly intended to remove excess fluid, but retain essential ions. It is most developed in freshwater Turbellarians, but reduced or absent in marine species, which do not have to remove excess water.

26. http://www.cartage.org.lb/en/themes/Sciences/Lifescience/GeneralBiology/http://www.cartage.org.lb/en/themes/Sciences/Lifescience/GeneralBiology/ Physiology/ExcretorySystem/Invertebrate/flatwormexcret.gif

27. Nervous system and sense organs • Flatworms possess a simple brain and one to five pairs of longitudinal nerve cords that are cross connected to form a ladder-like arrangement. • There has been a tendency towards reduction of the number of pairs of nerve cords and increased development of the ventral pair. A similar evolutionary pathway may have led to the development of the ventral nerve cord found in annelids and arthropods.

28. Nervous system of Dugesia http://biodidac.bio.uottawa.ca/ftp/BIODIDAC/ZOO/PLATYHEL/DIAGCL/TURB007C.GIF

29. Nervous system and sense organs • Neurons are specialized for different tasks e.g. sensory and motor functions, which is an important advance in the evolution of nervous systems. • There are a number of different sensory cells found in flatworms and tactile and chemoreceptive cells are abundant.

30. Nervous system and sense organs • In freshwater Planarians concentrations of sensory cells form two ear-like structures (the auricles) found on the side of the head. • Light sensitive eyespots or ocelli are common in all classes but Cestoda.

31. Freshwater Planarians: http://www.aecos.com/CPIE/flatworm.jpg

32. Reproduction • Reproduction in the Platyhelminthes can be asexual or sexual. However, most are hermaphroditic and cross fertilize. • In parasitic forms sexual and asexual reproduction may alternate in different stages of the life history

33. Classification of Platyhelminthes • There are four classes in the Platyhelminthes: • Class Turbellaria: free-living flatworms. • Class Trematoda: endoparasitic flukes • Class Monogenea: parasitic flukes that are mainly ectoparasites • Class Cestoda: tapeworms

34. Class Turbellaria • Class Turbellaria contains about 3000 species. There is considerable debate about the classification of the class and it is likely that the class is not monophyletic. • Most species are marine and benthic (move around on the bottom in aquatic environments). • Some also found in fresh water or in moist temperate and tropical terrestrial habitats.

35. 8.2 Marine turbellarian

36. Dugesia tigrina, a freshwater turbellarian © Mauricio A. Muñoz

37. Class Turbellaria • Most Turbellarians are predators of invertebrates smaller than themselves. Other species are herbivores or scavengers. • In many species the pharynx is protrusible and can be inserted into the prey to begin digesting it. • Turbellarians move by swimming, creeping or crawling. They combine muscular contractions with ciliary movement to move.

38. Class Turbellaria • Turbellarians’ outer surface is ciliated and the animal secretes mucus. Beating of cilia within this viscous mucus allows movement to occur. • Turbellarians may also use waves of muscle movement to move. The muscular contractions push and pull the animal forward. • To help grip the substrate when generating muscle contractions turbellarians use paired secretory glands on their underside to secrete first a viscous glue and later a second chemical which breaks the attachment to the surface.

39. “Polycladida moseleyi is distributed throughout the Mediterranean Sea and the temperate eastern Atlantic. Its favored food are tunicates (Clavelina sp.). “ http://www.rzuser.uni-heidelberg.de/~bu6/flat0431.html