Acoelomate Bilateral Animals: Features and Evolutionary Significance

1. Ch 10 Acoelomate Bilateral Animals

2. Acoelomate Bilateral Animals • Consist of phyla: • Phylum Platyhelminthes • Phylum Nemertea • And others

3. Reproductive and osmoregulatory systems Acoelomate Bilateral Animals • Simplest organisms to have bilateral symmetry • Triploblastic • Lack a coelom • Organ-system level of organization • Cephalization • Elongated, without appendages

4. Bilateral Symmetry • Divided along sagittal plane into two mirror images • sagittal= divides bilateral organisms into right and left halves

5. Anterior= head end • Posterior= tail end • Dorsal= back side • Ventral= belly side

6. Bilateral animals • Bilateral symmetry = important evolutionary advancement • Important for active, directed movement • Anterior, posterior ends • One side of body kept up (dorsal) vs. down (ventral)

7. Directed movement evolved with anterior sense organscephalization Cephalization • specialization of sense organs in head end of animals

8. Acoelomates lack a true body cavity • Solid body • no cavity b/w the digestive tract and outer body wall

10. Acoelomates are triploblastic • Triploblastic (3 germ layers) • Germ layer= layers in embryo that form the various tissues and organs of an animal body

11. 3 germ layers • Ectoderm • Outermost germ layer • Gives rise to outer covering of animal ie. epidermis • Endoderm • Innermost germ layer • Gives rise to inner lining of gut tract

12. Mesoderm • Middle germ layer • b/w ectoderm and endoderm • Gives rise to various tissues/organs (ie. muscles)

13. Acoelomate animals have an organ-system level of organization

14. Digestive tract and nervous system Acoelomate animals have an organ-system level of organization • Organ-system • Different organs operate together (ie. excretory system, nervous system) • mesodermal tissue gives rise to parenchyma

15. Polyclad • From Red Sea http://www.rzuser.uni-heidelberg.de/~bu6/

16. Phylum Platyhelminthes Flatworms Free living Parasitic

17. From Atlantic ocean http://www.rzuser.uni-heidelberg.de/~bu6/

18. Phylum Platyhelminthes • Flattened dorsoventrally • flatworms • 34,000 species • Gastrovascular cavity (if present) has only one opening (mouth = anus) • Mostly monoecious

19. Phylum Platyhelminthes • First phylum that has an Organ systems present • derived mesodermally (parenchyma): • Muscular system • Digestive system (incomplete; gastrovascular type) (absent in some) • Nervous system • Excretory system (absent in some) • Reproductive system

20. Rely on diffusion Phylum Platyhelminthes • Organ systems absent: • Circulatory • Respiratory

21. Hymenolepsis- rat tapeworm Phylum Platyhelminthes (cont’d) • Divided into 4 classes: • Class Turbellaria (mostly free-living flatworms) • Class Cestoda (tapeworms) • Class Trematoda (parasitic flukes) • Class Monogenea (parasitic flukes)

22. Class Turbellaria • Mostly free-living flatworms • Marine (mostly) or freshwater bottom-dwellers • Predators and scavangers • First group of bilateral symmetrical animals

23. Class Turbellaria (cont’d) • Move by muscles, ciliated epidermis

24. Class Turbellaria (cont’d) • Freshwater turbellarians adapted osmoregulatory structures • Protonephridia • protos= first • nephros= kidney • network of fine tubules running down sides of organism

25. Class Turbellaria (cont’d) • Flame cells= branch from tubules • Ciliary projections drive fluid down tubule • Tubules open to outside= nephridiopore

26. Class Turbellaria (cont’d) • nervous system with nerve ganglion • ganglion- aggregation of nervous tissue • Cephalization- cerebral ganglion= primitive brain

27. Class Turbellaria (cont’d) • Ocelli= light-sensitive eyespots

28. Reproductive and osmoregulatory systems Turbellarian Reproduction • Asexual (fission) • transverse • Sexual • Monoecious (mostly) • Cross-fertilization

29. All parasitic • lack cilia • Have unusual body covering: tegument Other 3 classes: • Class Trematoda • Class Cestoda • Class Monogenea • Outer zone of tegument (glycocalyx) • consists of proteins and carbohydrates • aids in transport of nutrients, waste, gases • Protection against host defenses

30. Class Trematoda • Parasitic flukes • Endoparasites • Hooks, suckers, increased reproductive capacity

31. 1mm-6cm long • Complex life cycle: • Definitive host (primary/final host) • where parasite matures and reproduces (sexually) (eggs released) • vertebrate

32. Intermediate host • Mollusc (ie. snail) • Hosts in which larval stages develop and undergo asexual reproduction • Results in an increase in the number of the individuals

33. Class Trematoda • Example: Chinese Liver Fluke

34. Chinese Liver Fluke • Infects 30 million people in eastern Asia • Lives in ducts of liver • Eats epithelial tissue, blood • Definitive host: • Humans, dogs, cats • 2 intermediate hosts: • snail • fish

35. Hymenolepsis- rat tapeworm Class Cestoda • Tapeworms • Endoparasites • Vertebrate host • Live in digestive tract • 1 mm- 25m long (EWWWW!!)

36. Hymenolepsis- rat tapeworm Class Cestoda • Highly specialized • Lack mouth, digestive tract • Absorb nutrients across body wall • Hooks and suckers • “head”= scolex

37. Adult tapeworms consist of long series of repeating units= proglottids • Chain of proglottids= strobila

38. Tapeworms are monoecious (mostly) • Mostly cross-fertilization • No specialized sense organs scolex

39. Cestodes depend on host digestion • Small molecules in host intestine, liver

40. Beef Tapeworm • Definitive host= human • Intermediate host= cattle

41. Class Monogenea • Parasitic flukes • Mostly ectoparasites • Single host, mostly fish

42. Phylum Nemertea • Triplobastic, acoelamate • bilateral symmetry • Unsegmented • Ciliated epidermis • Closed circulatory • usually <20cm • Marine mud, sand • Elongate, flattened worms

43. Phylum Nemertea (cont’d) • Unlike the platyhelminthes, Complete digestive tract, with anus • One-way • More efficient; allows larger growth

44. Phylum Nemertea (cont’d) • Cerebral ganglion, longitudinal nerve cords • Long proboscis used in carnivorous species • Two lateral blood vessels yet no heart • Dioecious • “two” “house” • Male and female organs in separate individuals