These notes provide a comprehensive summary of molluscs in a clear and easily understandable way. These notes include information from past exam papers and will enable you to succeed at zol1501
Phylum Platyhelminthes:
Members of this phylum are commonly called flatworms. Their bodies may be slender, broadly leaflike or long and
ribbonlike. They can be free-living or parasitic. Their external body covering is a syncytial tegument, or neodermis.
Characteristics of Phylum Platyhelminthes:
Free living or parasitic
Bilateral symmetry
Body flattened dorsoventrally
Triploblastic
Acoelomate body
Incomplete gut
Nervous system is a pair of anterior ganglia with longitudinal nerve cords
Sense organs include statocysts (organs of balance) and ocelli
Asexual reproduction
Mostly monoecious
Complex reproductive system
Internal fertilisation
Excretory system of 2 lateral canals with branches bearing flame cells (protonephridia)
Respiratory, circulatory and skeletal systems lacking
Form and Function:
Epidermis, Muscles:
- Most turbellarians have a cellular, ciliated epidermis resting on a basement membrane
- It contains rod shaped rhabdites which swell and form a protective mucous sheath around the body when
discharged. Single cell mucous glands open on the surface of the epidermis
- Most turbellarians have dual-gland adhesive organs in the epidermis.
- Other platyhelminthes have a non-ciliated body covering called a syncytial tegument instead of a ciliated
epidermis
- The term syncytial means many nuclei are enclosed within a single cell membrane
- Cell bodies communicate with the surface cytoplasm by sending extensions upwards. These extensions fuse
to form the syncytial covering
- Some shed their ciliated covering once a host is contacted
- The tegument is sometimes called the neodermis
- The tegument is resistant to the immune system of the host and has both absorptive and secretory functions
- In the body wall, muscle fibres run circularly, longitudinally and diagonally
- A meshwork of parenchyma cells developed from mesoderm fills the spaces between the muscles and
visceral organs
Nutrition and Digestion:
- Digestive system includes a mouth, a pharynx and an intestine
- In turbellarians the pharynx is enclosed in a pharyngeal sheath and opens just inside the mouth through
which it can extend
- Intestine with 3 many branched trunks. The whole forms a gastrovascular cavity lined with columnar
epithelium
- Mainly carnivorous. They detect food from some distance using chemoreceptors
- They entangle prey in mucous secretions
- Some can grip prey with anterior end, wrap around the prey and extend its pharynx to suck food
- Intestinal secretions contain proteolytic enzymes or some extracellular digestion
- Bits of food are sucked into intestine where phagocytic cells of the gastrodermis complete digestion
intracellularly
- Undigested food is egested through the pharynx
- Some have no digestive tract so they must depend on host digestion and absorption is confined to small
molecules from the host’s digestive tract
Excretion and Osmoregulation:
- Excess water is removed by osmoregulatory systems
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