Nerve net with longitudinal
nerve fibres with transverse connections
They are aquatic, terrestrial
Soft-bodied and worm-like
About 25,000 living
species have been described. They have true bilateral symmetry, a cellular
layer (parenchyma) between the epidermis and the gastrodermis, and dorsoventral
flattening of the body. They have no blood or circulatory system.
Respiration is by diffusion,
and this constrains body size in at least one dimension. The larger species are
either ribbon-shaped or leaf-shaped, see Eurylepta rubrocincta class Turbellaria. They range in size from less than one millimetre to over one metre in
length. Most are parasitic and have evolved various specialized attachment
organs; the free living specimens have ventral cilia to aid in locomotion. They
are subdivided into four classes;
The Trematoda are
endoparasitic with a complicated life-cycle. They are mainly parasites of vertebrates (the definitive host); the most common intermediate hosts are Molluscs. There are around 12 000 species, and the adult size range is 1 cm - 6 m. Their epidermis is syncitial as in the Monogenea (see below), and they
have various suckers, hooks and attachment organs; the most common arrangement
being an oral and mid-ventral sucker, as is found in the liver fluke.
Trematodes of the Schistomatidae family have separate sexes (unusual for
Platyhelminthes). The males are shorter and fatter than the females, and have a
ventral groove (gynecophoric canal) along their body into which the female can
fit. Schistosoma mansoni (left) causes the disease shistosomiasis which infects around 200 million people world-wide, causing body pain, anaemia and dysentery, and kills over 250 000 every year. It is fairly common in
wet-rice fields. The fertilized eggs break into the host's bladder and pass out
with the urine into the soil. The larva, called a miricidium hatches and enters
the body of a snail. Within the snail's body cavity it grows into a large sac
called a sporocyst, and this then hatches into a further larval stage called
the redia. The redia has a mouth and gut and migrates to the snail's digestive
gland. The redia develops into a tiny version of the adult form called a
cercaria which drills out of the snail and into water where it enters the human
either through the skin or mouth. The cercaria of another species of
schistosomatidae occasionally burrows into human skin only to die and cause
what is commonly known as "swimmer's itch".
The common sheep
liver fluke, Distomum hepaticum or Fasciola hepaticum has the dwarf pond snail as its
intermediate host, and is another common trematode. The adult fluke lives in the bile passage in the liver of sheep, other ruminants, or even man. The eggs are passed with the faeces. After hatching the enter the snail. Pass out of the snail and encyst on vegetation, which is eaten by the definitive host.
The Monogenea and the Trematoda
(above) are commonly known as flukes. There are around 1100 species. They differ
in two main characteristics. The Monogenea are mainly ectoparasites of fish and
have a simple life-cycle involving only one host; whereas the Trematoda are
mainly endoparasitic and have a more complicated life-cycle involving more than
The Monogenea have an attachment organ, usually a sucker called the opisthaptor (see Gyrodactylus cylindriformis, below, around the
mouth and a posterior attachment organ that can also be a sucker, or hooks or
clamps. Their epidermis is syncitial (many nuclei with no, or little,
separation between cells). Monogenea larvae are free-swimming.
above the scolex (head) of a tapeworm
These are commonly
known as the tapeworms, and are adapted for parasitizing vertebrate
guts. There are around 3,500 species described. They are usually less than 1 mm thick but some individuals such as Taenia sp. (below right) can be as long as 30 m. The intermediate host can be a vertebrate or invertebrate according to
species. In the vertebrate gut they are surrounded by food so have no need for
a mouth or gut of their own. Nutrients are absorbed directly through the
surface which has many folds increasing the surface area, and is equipped with
a large number of mitochondria. Cestodes have an attachment organ known as a scolex (see left) that is equipped with suckers and spiny hooks with which it attaches itself to its host's intestinal wall. Behind the scolex the
body is formed of segments (proglottids), so the oldest segments are at
the posterior end.
Tapeworms are hermaphroditic and the end segments are
essentially just bags of eggs that have been fertilized by sperm from the same
segment, a different segment of the same worm, or a different worm. In the
photograph on the right of Taenia sp. the genital pore, which
shows up as a bulge on the side of the proglottid can be seen in some. There
can be as many as 1000 proglottids in each worm and 100 000 eggs in each gravid
proglottid. The proglottids regularly mature and detach and are shed with the faeces of the host. Cows and pigs can act as the intermediate host to Taenia
sp., and can infect man when meat is not properly cooked. Dipylidium sp. infect cats and dogs, and occasionally man. The
intermediate host is the flea, and the tapeworm passes into the cat or dog as
they clean themselves by licking and biting. Tapeworms can lead to nutritional deficiencies in the host.
Tapeworms found in humans
Taeniarhynchus saginatus (beef tapeworm). Infection can come from eating raw or rare beef. This is the most common tapeworm in humans.
Taenia solium (pork tapeworm). Infection can occur when rare or improperly cooked pork is eaten, and can cause epilepsy if the worm lodges in the brain. This is called cysticercosis, and in Los Angeles is the cause of 10% of seizure cases taken to emergency rooms. Cysticercosis affect 41,000 - 169,000 people in the USA. Diphyllobothrium latum (fish tapeworm).
infection can occur when raw or improperly cooked fish is eaten. Infection is common in countries where raw fish is eaten, infection is also fairly common around the Great Lakes of North America.
Diplydium caninum (dog tapeworm). Infection occurs due to unhygienic habits around dogs who have fleas or lice.
Vampirolepis nana (dwarf tapeworm). The intermediate host is the flour beetle, and infection in humans is fairly common.