Phylum: Platyhelminthes

• Characterised by their flat bodies, flattened dorsoventrally. Known as flat worms
• Distinguish between free-living species that live in water, and parasites that live in the body of a host
• Has on organ level of organisation, tissues are organised to form organs, these animals are still primitive
• Three germ layers and are therefore triploblastic
• Ectoderm gives rise to outer, protective layer, the epidermis, nervous system and excretory organs
• The endodermis forms lining of digestive tract that plays a role of digestion and absorption
• The mesoderm forms parenchyma cells that fill the space between the digestive tract and the epidermis
• No cavity, therefore flatworms are acoelomate
• Internal organs, reproductive system and connective tissue occur in the parenchyma of the mesoderm
• The presence of a mesoderm enables a more complex way of life
• Flatworms have a definitive anterior end(head), where nerves and sensory organs are concentrated. This is cephalisation
• Cephalisation is associated with bilateral symmetry and facilitates locomotion
• The epidermis can be covered with cilia or a cuticle (in parasites)
• Movement is brought about by muscles in the parenchyma, and locomotion with the help if cilia (in free-living flatworms)
• Food is animal matter
• Ingestion occurs through the mouth of through absorption of digested nutrients through the body wall (parasites)
• Digestion, if necessary, occurs in the cells of the endoderm that line the digestive tract
• Egestion occurs through the mouth
• They do not have a through gut, bot only one opening, the mouth
• No specialised transport system is present
• In some flatworms substances are transported to all parts of body via a highly branched digestive canal
• Body is elongated and flattened, so cells are close to the surface
• Gases are transported by diffusion
• Excretion os metabolic waster products occurs through specialised cells, the flame cells