Flatworms, Roundworms, and Plant Structures
Platyhelminthes (Flatworms)
Introduction – Flatworms have bilateral symmetry.
Bilateral Symmetry – Symmetry that divides the body of a living being into two equal parts from a longitudinal section.
Habitat – Flatworms are found in:
- Wet soil – Geoplana
- Freshwater – Dugesia
- Saltwater – Notoplana and Stylochus
- Inside living organisms, as parasites – Taenia, Schistosoma, Fasciola
Structure – Flatworms are elongated animals with a flattened dorsoventral body, segmented or unsegmented. They have digestive and excretory structures but lack respiratory or circulatory systems.
Nutrition – They have an incomplete digestive system and undergo mixed digestion (extracellular and intracellular).
Nervous System – They have two nerve ganglia called cerebral ganglia in the head, below the ocelli.
Excretion – They possess flame cells (solenocytes) for the first time.
Reproduction – Can be sexual or asexual. Asexual – regeneration in planarians and strobilation in tapeworms. Sexual – mainly internal and cross-fertilization, although many are hermaphroditic.
Classification – Three classes:
Turbellarians
This class consists exclusively of free-living animals like flatworms (freshwater) and Geoplana (land).
Trematodes
All species are parasites. They have branched mouths and intestines, with two suckers for attachment (oral and ventral).
Tapeworms
All are parasites. They have a distinct head followed by segments called proglottids.
Verminoses (Diseases caused by flatworms)
Schistosomiasis (Water Belly)
Caused by Schistosoma mansoni. Transmitted in areas with poor sanitation, carried by freshwater rivers.
Taeniasis
Caused by Taenia saginata and Taenia solium (solitary worms). Transmitted through undercooked meat containing larvae, leading to cysticercosis.
Cysticercosis
Caused by T. solium. Transmitted by ingesting food contaminated with T. solium eggs.
Aschelminthes (Roundworms)
Introduction – Considered by many taxonomists as a collection of independent groups, including roundworms or nematodes.
Habitat – Free-living in wet soils, freshwater, and sea, or parasitic in animals or plants.
Structure – Elongated body, tapering at the ends, unsegmented, with bilateral symmetry, covered by a protective cuticle.
Nutrition – Complete digestive system with an anal opening (first in the zoological scale). Digestion is extracellular in free-living forms and absent in intestinal parasites.
Nervous System – Centralized with a brain-shaped ring.
Excretion – By a giant H-shaped cell.
Reproduction – Generally separate sexes with sexual dimorphism. Fertilization is internal and crossed, with high egg production. Development can be direct or indirect. Asexual reproduction is rare.
Diseases caused by Aschelminthes
Ascariasis (Worms)
Caused by Ascaris lumbricoides. Acquired by eating raw vegetables or water contaminated with eggs.
Hookworm (Yellowing)
Caused by Ancylostoma duodenale and Necator americanus. Acquired by larval penetration through the skin.
Elephantiasis
Caused by Wuchereria bancrofti. Transmitted by Culex mosquitoes.
Plant Morphophysiology
Plant Growth
Seed transformation into a complete plant involves division, expansion, and differentiation.
Roots
Roots exhibit positive geotropism, growing towards the earth’s center. They lack chlorophyll and function to anchor the plant and absorb water and minerals.
Suberosa Region
Produces secondary roots, which develop tertiary roots, forming the root system.
Root Hair Zone
Characterized by root hairs from epidermal cells, absorbing water and minerals.
Smooth Region
Lacks any attached structures.
Root Cap (Calyptra)
Protective covering of meristematic tissue at the root apex.
Stem
Forms from the embryo’s caulicle growth and differentiation.
Nodes
Sites of intense cell division and differentiation, forming new structures.
Internodes
Intervals between nodes.
Buds
Regions of meristematic tissue forming stem buds.
Leaves
Responsible for photosynthesis.
Blade (Limbo)
Thin, green surface with innervations.
Petiole
Connects the blade to the stem node.
Sheath
Region at the base of the petiole.
Meristem
Tissue with permanent embryonic characteristics.
Primary Meristem
Apical Meristem
Located at the stem apex, promoting longitudinal growth.
Root Apical Meristem
Delicate embryonic cells covered by the root cap.
Lateral Buds
Primary meristems giving rise to lateral branches.
Secondary Meristem
Phellogen
Located at the periphery of the stem and root, producing cork and phelloderm.
Interfascicular Cambium
Located in the medullary region, producing conducting tissues.