Phylum Platyhelminthes Overview

Phylum Platyhelminthes

• Flatworms are dorsoventrally compressed with bilateral symmetry.
• They are triploblastic.
• The epidermis is ciliated in some species (e.g., Turbellaria), while others have a tegument (e.g., Monogenea, Digenea, Cestoda), potentially with suckers or hooks for attachment to a host.
• They lack a secondary body cavity (acoelomate).
• The muscular system is well-developed.
• They have an incomplete digestive system (mouth but no anus).
• Their excretory system includes flame cells or solenocytes.
• They possess some sense organs and a central nervous system.
• Most are hermaphroditic (except Schistosoma).
• They lack skeletal, circulatory, and respiratory systems.
• The phylum is divided into 5 main classes: Turbellaria (mostly free-living), Monogenea (ectoparasitic), Aspidogastrea, Digenea (endoparasitic), and Cestoda (tapeworms, endoparasitic, segmented).

Class Turbellaria (Order Tricladida - Dugesia (Planaria))

• Mostly free-living flatworms.

Class Digenea

• All are endoparasitic.
• Their bodies are flattened and unsegmented.
• They attach to host tissues using suckers.
• They have a complex muscular system and lack an anus.
• They possess excretory organs and a nervous system.
• Infections are classified by location (Fasciola - liver flukes, Schistosoma - blood flukes, Heterophyes - intestinal flukes).

Class Cestoda

• These are tapeworms or gutless flatworms.
• Their bodies are elongated and segmented.
• They have a head (scolex) with suckers, hooks, or bothria for attachment.
• The body consists of proglottids, which are reproductive segments.
• They have three types of proglottids: immature, mature, and gravid.
• They lack a digestive system and absorb nutrients through their tegument.
• They have a nervous system composed of nerve bundles and nerve cords.
• Their excretory system includes ducts and pores.
• They reproduce through cross-fertilization or self-fertilization and have a distinctive life cycle.
• Key examples: Taenia saginata (beef tapeworm) and Taenia solium (pork tapeworm)

Phylum Nematoda

• These are roundworms or threadworms.
• They are bilaterally symmetrical, unsegmented, and triploblastic.
• They are pseudocoelomate, meaning they have a body cavity that's not completely lined with mesoderm.
• They have a complete digestive system with a mouth and an anus.
• They lack respiratory and circulatory systems.
• Their body wall has a cuticle and longitudinal muscles.
• They have an excretory system of canals or glandular organs.
• Most are dioecious (separate sexes), and fertilization is internal.
• They have nerves & a nervous system with anterior and posterior rings and nerves.
• Their eggs have a chitinous shell.
• They typically undergo direct development (no larval stages).
• Classified into two main classes: Secernentea (includes worms like Ascaris lumbricoides, Ancylostoma duodenale, and Enterobius vermicularis) and Aphasmidea(includes worms like Trichinella spiralis).

Ascaris lumbricoides

• A common intestinal parasite of humans and other animals.
• Causes ascariasis, a disease that ranges from mild abdominal discomfort to severe complications (obstruction, hepatitis).
• Infection occurs through ingestion of contaminated food or water, containing infective larvae.
• Ascaris has characteristic anterior and posterior morphology.

Ancylostoma duodenale

• Causes hookworm disease (ancylostomiasis)

Taenia saginata and T. solium

• Intestinal parasites in humans.
• Infections are caused by eating undercooked meat, containing the infective stage (bladder worms).
• Causes taeniasis, which can lead to mild anemia, digestive and nervous disorders.