Flatworms Copyright © McGraw-Hill Education Body Structure Flatworms are acoelomates with bilateral symmetry. Belong to phylum Platyhelminthes Thin, flat, ribbon-like bodies Definite head region and body organs Most flatworms are parasitic, but some are free-living

Flatworms Copyright © McGraw-Hill Education Body Structure Feeding and digestion Free-living flatworms feed on dead or slow-moving organisms. Extend a tubelike pharynx out of their mouth which releases digestive enzymes

Flatworms Copyright © McGraw-Hill Education Body Structure Feeding and digestion Parasitic flatworms have modified feeding structures called hooks and suckers, which enable them to stay attached to their hosts. Some have reduced digestive systems, others lack digestive systems and gain nutrients through direct absorption from the host’s body.

Flatworms Copyright © McGraw-Hill Education Body Structure Respiration, circulation, and excretion Flatworms do not have circulatory or respiratory organs. Cells can use diffusion to move dissolved oxygen and nutrients Flatworms do have an excretory system that consists of a series of small tubes. Flame cells lined with cilia sweep water and excretory substances into tubules Also excrete through their mouths

Flatworms Copyright © McGraw-Hill Education Body Structure Response to stimuli Most flatworms have a nervous system consisting of two nerve cords with connecting nerve tissue. Flatworms have several anterior ganglion at the end of the nerve cord, groups of nerve cell bodies that coordinate incoming and outgoing nerve signals.

Flatworms Copyright © McGraw-Hill Education Body Structure Movement Move by contracting muscles in the body wall Glide by using cilia located on their undersides

Flatworms Copyright © McGraw-Hill Education Body Structure Reproduction Flatworms are hermaphrodites. Two different flatworms exchange sperm, and the eggs are fertilized internally. Free-living flatworms can reproduce asexually by regeneration, where lost body parts can regrow whole organisms.

Flatworms Copyright © McGraw-Hill Education Diversity of Flatworms Turbellarians Class Turbellaria consists of free-swimming flatworms Have eyespots, can detect presence/absence of light Have chemical detecting cells Trematodes Class Trematoda consists of parasitic flukes Often require two hosts

Flatworms Copyright © McGraw-Hill Education Diversity of Flatworms Cestodes Class Cestoda contains all tapeworms Anterior end composed of a scolex, a knob-shaped structure with hooks and suckers for attaching to the intestine of a host Body is composed of individual sections called proglottids, each of which contains muscles, nerve cells, flame cells, and reproductive organs.