Helminthes are eukaryotic true multicellular organisms.
Adult helminthes are not always microscopic, but their larvae and eggs are. For this reason, they are subjects of microbiology. The
Trematoda
is a
Class
within the
Phylum

Platyhelminthes
, which contains two groups of parasitic worms , commonly referred to as flukes .
Nearly all trematodes are parasites of molluscs and vertebrates. The smaller
Subclass

Aspidogastrea
, comprising about 100 species, is obligate parasites of molluscs and may also infect turtles and fishes. The other
Subclass
Digenea
, which constitutes the majority of trematode diversity, is obligate parasites of both mollusks and vertebrates, but do not occur in cartilaginous fishes.
Use of the term "fluke" in relation to human infection refers solely to digenean infections.
Characteristic features of the
Digenea
include a tegument. They possess a vermiform (worm like), unsegmented body­plan. The features of a worm­like animal include bilateral symmetry, leglessness and soft­bodies with a length greater than two or three times its breadth There are typically 2 suckers , an anterior oral sucker surrounding the mouth, and a ventral sucker sometimes termed the acetabulum , on the ventral surface. Monostome is a term used to describe worms with one sucker (oral).
Flukes
with an oral sucker and an acetabulum at the posterior end of the body are called
Amphistomes
.
Distomes
are flukes with an oral sucker and a ventral sucker, but the ventral sucker if somewhere other than posterior.
Adult digeneans are commonly hermaphroditic . The flukes can be classified into two groups, on the basis of the system which they infect.
Tissue flukes are species, which infect the bile ducts, lungs, or other biological tissues. This group includes the lung fluke,
Paragonimus westermani
, and the liver flukes
,
Clonorchis sinensis and
Fasciola hepatica
.
The other group are known as blood flukes
, and inhabit the blood in some stages of their life cycle. Blood flukes include various species of the genus
Schistosoma
.
Trematodes have a complex life cycle, often involving several hosts.
The eggs pass from the host with the feces. When the eggs reach water, they hatch into free­swimming forms called miracidia. The miracidia penetrate a snail or other molluskan host to become sporocysts. The cells inside the sporocysts typically divide asexually by mitosis to form rediae .

Rediae, in turn, give rise to free­swimming cercariae, which escape from the mollusk into water. Using enzymes to burrow through exposed skin
, cercariae penetrate another host (often an arthropod ) and then encyst as metacercariae.
When this host is eaten by the definitive host, the metacercariae excyst and develop, and the life cycle repeats.

Fasciola hepatica

Paraginimus westwrmanii

Cestodes are tapeworms:
Phylum Platyhelminthes
;
Class Cestoidea
;
Subclass Cestoda (beltlike
)

The tapeworms belong to another branch of the
Platyhelminths
(flat worms) known as the
Cestodes
. They are flat in cross­section; they are entirely parasitic and are hermaphroditic . The tapeworms are highly specialized parasites.
Many of their organ systems have disappeared in their millions of years of parasitic existence, and there is no free­living organism, which even closely resembles them. The common ancestor, whom the tapeworms had with the other platyhelminths, has been lost to time.
Adult tapeworms live only in intestines. Adult tapeworms “body”, has an anchoring organ or scolex , which attaches them to the intestinal wall. The scolex may be armed with suckers, hooks, both or neither to help it hold fast. Growing out behind the scolex are the segments named proglottids ­ repeating units that form strobila , the segmented main body part of the adult