Acoelomate Bilateral Animals

1. Acoelomate Bilateral Animals

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

4. 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

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

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

7. 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)

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

9. Acoelomate Phylum Platyhelminths Or not shown here Nemerterean • Acoelomates lack a true body cavity • Solid body • no cavity b/w the digestive tract and outer body wall This is a round worm Different Phylum

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 Trematoda (parasitic flukes) • Class Monogenea (ectoparasitic flukes) • Class Cestoda (tapeworms)

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

23. Digestion is completed within the cells lining the gastro- vascular cavity, which has three branches, each with fine subbranches that pro- vide an extensive surface area. Pharynx. The mouth is at the tip of a muscular pharynx that extends from the animal’s ventral side. Digestive juices are spilled onto prey, and the pharynx sucks small pieces of food into the gastrovascular cavity, where digestion continues. Undigested wastes are egested through the mouth. Gastrovascular cavity Eyespots Ganglia. Located at the anterior end of the worm, near the main sources of sensory input, is a pair of ganglia, dense clusters of nerve cells. Ventral nerve cords. From the ganglia, a pair of ventral nerve cords runs the length of the body. Figure 33.10 • The best-known turbellarians, commonly called planarians • Have light-sensitive eyespots and centralized nerve nets

24. Class Turbellaria (cont’d) • Move by muscles, ciliated epidermis /gastrovascular cavity

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

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

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

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

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

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

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

32. 1mm-6cm long • Complex life cycle: • Definitive host (primary/final host) • where parasite matures and reproduces (sexually) (eggs released) • vertebrate

33. 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

34. Mature flukes live in the blood vessels of the human intestine. A female fluke fits into a groove running the length of the larger male’s body, as shown in the light micrograph at right. 1 Male Female 1 mm 5 These larvae penetrate the skin and blood vessels of humans working in irrigated fields contaminated with infected human feces. Blood flukes reproduce sexually in the human host. The fertilized eggs exit the host in feces. 2 3 The eggs develop in water into ciliated larvae. These larvae infect snails, the intermediate hosts. Asexual reproduction within a snail results in another type of motile larva, which escapes from the snail host. 4 Snail host Figure 33.11 • Trematodes that parasitize humans • Spend part of their lives in snail hosts

35. 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

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

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

38. Proglottids with reproductive structures 200 µm Hooks Scolex Sucker Figure 33.12 Tapeworm • Tapeworms • Are also parasitic and lack a digestive system

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

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

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

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

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

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

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

46. Nemertea

47. 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

48. Sea serpents?