Classification of Phylum Platyhelminthes:
o No clear defining feature
o In marine, freshwater, and moist terrestrial habitats
o Turbellarian flatworms are mostly free-living; classes Monogenea,
Trematoda, and Cestoda entirely parasitic
o Bilaterial symmetry; definite polarity of anterior and posterior
ends; body flattened dorsoventrally
o Adult body three-layered (tripoblastic)
o Body acoelomate
o Epidermis may be cellular or syncytial (ciliated in some);
rhadbites in epidermis of most Turbellaria; epidermis a synticial
tegument in Monogenea, Trematoda, Cestoda, and some
o Gut incomplete, may be branched, absent in cestodes
o Muscular system primarily of a sheath form and of mesodermal
orgin; layers of circular, longitudinal, and sometimes oblique
fibers beneath the epidermis
o Nervous system consisting of a pair of anterior ganglia with
longitudinal nerve cords connected by transverse nerves and
located in the mesenchyme in most forms
o Sense organs include statocysts (organs of balance) and ocelli
o Asexual reproduction by fragmentation and other methods as
part of complex life cycles
o Most forms monoecious; reproductive system complex, usually
with well-developed gonads, ducts, and accessory organs;
internal fertilization; development direct in free-swimming forms
and those with single hosts; complicated life cycle often
involving several hosts in many internal parasites
o Excretory system of two lateral canals with branches bearing
flame cells (protonephridia); lacking in some forms
o Respiratory, circulatory, and skeletal systems lacking; lymph
channels with free cells in some trematodes.
Platyhelminthes are divided in 4 classes: Turbellaria, Trematoda, Monogenea
Class Turbellaria contains the free-living flatworms, along with some
symbiotic and parasitic forms
Most Turbellarians are bottom dwellers in marine or freshwater, living
under stones or other hard objects.
All members of classes Monogenea, Trematoda (flukes), and Cestoda
(tapeworms) are parasitic. Most Monogenea are ectoparasites, but all
trematodes and cestodes are endoparasitic. Many species have indirect life cycles with more than one host; the first host is often an
invertebrate, and the final host is usually a vertebrate. Humans serve as
hosts for a number of species.
Form and Function – Epidermis, Muscles:
Most turbellarians have a cellular, ciliated epidermis resting on a
basement membrane. It contains rod-shaped rhadites, 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
orders of turbellarians have dual-gland adhesive organs in the
epidermis. These organs consist of 3 cell types: viscid and releasing
gland cells and anchor cells. Secretions of the viscid gland cells
apparently fasten microvilli of the anchor cells to the substrate, and
secretions of the releasing gland cells provide a quick, chemical
In contrast to the ciliated cellular epidermis of most tubellarians, adult
members of the 3 parasitic classes have a nonciliated body covering
called a syncytial tegument.
The term syncytial means that many nuclei are enclosed within a single
cell membrane. It might appear that a completely new body covering
appeared in the parasitic classes, but there are some free-living
turbellarians with an atypical epidermis.
Adults of all members of Trematoda, Monogenea, and Cestoda
possess a syncytial covering that entirely lacks cilia and is designated a
The larval forms of many of these groups are ciliated, but the ciliated
coverage is shed once a host is connected. Epidermal shedding has
been suggested as a means of avoiding host immune response.
Development of tegument occurs as several surface layers of the
epidermis are shed; eventually fused cytoplasmic extensions from cell
bodies below the basement membrane become the surface covering
of the body. The tegument is sometimes called the neodermis, and its
shared presence among the parasites is the basis for uniting
trematodes, monogeneans, and cestodes in clade Neodermata.
The tegument is resistant to the immune system of the host in
endoparasites, and it resists digestive juices in tapeworms and others
that dwell in a host gut. The syncytial nature of the tegument might
render it more resistant because there are no penetrable junctions
between cells. The tegument can be both absorptive and secretory.
Tapeworm tegument absorbs nutrients from the host’s digestive cavity
– tapeworms have neither mouth nor gut.
In the body wall below the basement membrane of flatworms are
layers of muscle fibers that run circularly, longitudinally, and diagonally.
A meshwork of parenchyma cells, developed from mesoderm, fills the spaces between muscles and visceral organs. Parenchyma cells in
some, perhaps all, flatworms are not a separate cell type but are the
non-contractile portions of muscle cells.
Nutrition and Digestion:
In general, Platyhelminth digestive systems include a mouth, a pharynx,
and an intestine. In turbellarians, such as the planarian, the pharynx is
enclosed in a pharyngeal sheath and opens posteriorly just inside the
mouth, through which it can extend. The intestine has 3 many-
branched trunks, one anterior and two posterior. The whole forms a
gastrovascular cavity lineds with columnar epithelium.
Planarians are mainly carnivorous. They can detect food from some
distance by means of chemoreceptors. They entangle prey in mucous
secretions from the mucous glands and rhadbites. A planarian grips
prey with its anterior end, wraps its body around the prey item, extends
it pharynx, and sucks up food in small amounts.
Intestinal secretions contain proteolytic enzymes for extracellular
digestion. Bits of food are sucked into the intestine, where phagocytic
cells of the gastrodermis co