Radial vs. Bilateral Symmetry

Cephalization Bilateral Symmetry usually has led to cephalization—the process by which sensory organs and appendages became localized in the head end of animals.

Evolutionary Trends If we analyze the basic body plans of animals, we find that they illustrate evolutionary trends. Four major “advances” (in order): Multicellular body plan (tissues) Bilaterally symmetrical body plan “Tube-within-a-tube” body plan Coelomate body plan

3 Major Bilateral Body Plans Acoelomates Pseudocoelomates Coelomates Each plan consists of 3 cell layers: endoderm, mesoderm, ectoderm

Body Cavities http://www.youtube.com/watch?v=AJ_qZ6ld6A0

Acoelomates These animals have no other cavity than the gut. They are often called the “solid worms.”

Pseudocoelomates These animals have a body cavity (the pseudocoelom) which is not completely lined with mesoderm. The “tube within a tube” body plan. This category is also composed of mostly worms.

Coelomates These animals have a “true coelom” lined with mesodermal peritoneum. Most animals are coelomate.

Platyhelminthes

Introduction to Planaria http://www.youtube.com/watch?v=w0QzSYQGsnA

Flattened Body, one opening

Introduction to Flatworms Phylum Platyhelminthes: platy = flat, helminth = worm Simplest animals with bilateral symmetry Body has 3 cell layers ectoderm (outer) mesoderm (middle) endoderm (inner) Show the beginnings of cephalization (i.e. a head) Can be free-living or parasitic ectoderm mesoderm endoderm gastrovascular cavity

Bilateral Symmetry

Sexual or Asexual Reproduction

Form and Function of Flatworms Digestive system: Gastrovascular cavity where food is broken down Two-way digestive system Food and wastes enter/exit the pharynx e.g.Planaria Pharynx = opening to the gastrovascular cavity

Respiratory system: Circulatory system: Excretory system: Flame cells - primitive excretory structures that get rid of excess water Metabolic wastes diffuse through the skin No organized systems for these O2, CO2 diffuse through the ectoderm

Planaria nervous system Fairly well-developed Has primitive “brain” and 2 ventral nerve cords Ocelli (aka eyespots) to detect light/dark Two auricles sensitive to touch and chemicals Musculoskeletal system: Muscles and cilia on ventral surface for locomotion auricle Planaria nervous system

If you cut a part off, it grows into a new worm! http://www.youtube.com/watch?v=wn3xluIRh1Y Free-living forms are hermaphrodites Asexual reproduction - fission/regeneration Sexual reproduction - exchanging sperm between 2 worms Parasitic worms reproduce sexually only Each worm produces huge numbers of sperm and eggs If you cut a part off, it grows into a new worm!

Planaria reproductive system

Budding

Some are parasites

Tape worm mouth parts

Class Monogenea Parasitic flatworms with one main host Most infect the gills of fish

Classes of Flatworms Free-living, usually small, freshwater Class Turbellaria Free-living, usually small, freshwater Predators or scavengers Example member: Planaria

Classes of Flatworms 2) Class Trematoda (flukes) Parasitic (internal parasites)  often infect blood and organs Often have more than 1 host Reduced digestive and nervous systems Example member: blood flukes (Schistosoma); has 2 hosts (p. 687) humans = primary host snails = intermediate host

Class Trematoda - blood fluke Schistosoma primary host: human Intermediate host: snail

Classes of Flatworms 3) Class Cestoda (tapeworms) Long and flat parasites Head is called scolex Narrow neck region that divides to form proglottids (contain huge numbers of sperm and eggs) (beef tapeworm) No digestive (absorb through skin- tegument) or nervous systems Highly developed reproductive system proglottid scolex

Cestode (tapeworm) l.c.