Ch 10 Acoelomate Bilateral Animals Acoelomate Bilateral Animals

Ch 10 Acoelomate Bilateral Animals

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

Acoelomate Bilateral Animals 1. Simplest organisms to have bilateral symmetry 2. Triploblastic 3. Lack a coelom 4. Organ-system level of organization 5. Cephalization 6. Elongated, without appendages Reproductive and osmoregulatory systems

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

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

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)

Directed movement evolved with anterior sense organs cephalization Cephalization – specialization of sense organs in head end of animals

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

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

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

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

Acoelomate animals have an organ-system level of organization

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

Polyclad • From Red Sea http: //www. rzuser. uni-heidelberg. de/~bu 6/

Phylum Platyhelminthes Flatworms Free living Parasitic

• From Atlantic ocean http: //www. rzuser. uni-heidelberg. de/~bu 6/

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

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

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

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) Hymenolepsis- rat tapeworm

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

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

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

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

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

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

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

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

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

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

2. 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

• Class Trematoda – Example: Chinese Liver Fluke

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

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

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

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

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

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

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

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

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

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

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