Trematode Life Cycles

1. Life Cycles of Digenetic Trematodes
Chapter 15, p. 230-237
Digenetic trematodes use 2 or 3 hosts in their life cycles.

FIRST INTERMEDIATE HOST is always a ___________________________.

Reproduction ?
SECOND INTERMEDIATE HOST may occur in some life cycles. This host
is an _____________________________________________________
Reproduction?
DEFINITIVE HOST is a _________________________________.
Reproduction?

2. Life Cycles of Digenetic Trematodes
As many as _____________________________________________ have been
recognized in digenetic trematode life cycles.
- Many variations occur - not all 6 larval stages may be present
within a species.
- Life cycles have been difficult to elucidate due to all these
larval stages.

Today I will discuss a generalized trematode life cycle.

We will examine the specific life cycle of each trematode of medical and
veterinary importance when we get to each one in lecture and lab.

3. Generalized Life Cycle

4. Adult
ADULT occurs in the definitive host
Although most adults are _____________________________.
Method of fertilization?
Sexual reproduction involving gamete formation (oocytes and
sperm) followed by fertilization results in the formation of
thousands of fertilized eggs.

5. Egg = Ovum
After fertilization and eggshell formation in the ootype, eggs move through
the uterus.
As they move through the uterus, 2 processes occur:
1. ______________________________ - eggshell is hardened into a tough
protein called ______________________ as the egg passes along the uterus.
- function?

6. Egg = Ovum
2. ________________________________________- mitosis of the zygote
to form the embryo.
- If the embryo is fully differentiated into a miracidium before
the egg is released from the adult, an
_________________________________is produced.
- If the embryo consists of a number of undifferentiated cells
when the egg is released, an
_________________________________is produced.
embryonated egg unembryonated egg

7. Egg = Ovum

Eggs are ejected from the adult by the
_____________________________
through the common genital pore.
Eggs leave the definitive host via
___________________________________________
(depending on the species) and are generally
deposited into an aquatic environment.
Hatching of the egg may occur:
1.
or
2.
Hatching of the egg involves the
opening of an ______________________________

8. Miracidium
Ciliated embryo that hatches from the egg
Miracidia that hatch in water:

In some species, the egg is ingested by the snail
and the miracidium hatches within the snail:

9. Miracidium
Morphology: Small, elongate organism covered
by __________________________

At anterior end is the
_________________________ that functions in
contacting the snail intermediate host.

The apical papilla contains openings from
_____________________________________.
When a snail is contacted, the penetration glands
secrete enzymes to aid in embedding the miracidium
within the snail's tissues.

___________________________ are common for
photoreception.

A mass of ______________________________ lies
in the posterior end. These cells divide asexually to
form the next larval generation.

10. Miracidium
Upon entering a snail, the miracidium loses its cilia and transforms into a sac-like
body called the __________________________________________

11. Sporocyst
Upon entering a snail, the miracidium loses
its cilia and transforms into a sac-like body
called the MOTHER SPOROCYST.
Mother sporocysts occur in the snail at the
site of penetration.
Mother sporocysts consist of a
_______________________________
surrounding developing
_______________________________
mouth or digestive tract?

12. Sporocysts
In some digenetic trematodes, the
germ cells in the mother sporocyst
divide asexually to give rise to
another generation of sporocysts
called ________________________.
Mother sporocyst bursts to release
these daughter sporocysts.

In other trematodes, the germ cells of
the mother sporocyst form a new
larval stage called the
______________________________

13. Redia
Mother rediae burst out of the
mother sporocyst and migrate to the
____________________________
____________________________
__of the snail host.

hepatopancreas

14. Redia
Each redia contains a
__________________________
and short
___________________________
What do they feed on?

________________________ in the
mother rediae may develop into a
second generation of rediae called
_____________________________
or may develop into
_____________________________.

15. Cercaria
CERCARIAE leave the sporocyst or
redia, migrate out of the snail, and
become __________________________.
Morphology:
1. Cercarial body is an
______________________________
- oral sucker and acetabulum are present
- digestive tract is complete
- excretory system is developed
- many types of gland cells are present
- 2 eyespots are common
- no reproductive structures

Furcocercous
cercaria
Microcercous
cercaria

16. Cercaria
2. _____________________________
used for swimming.
- may be unbranched with or without fins
- tail characteristics important in
identification
Type of cercaria is based on characteristics
of the body and tail. We will look at 3
types:
Furcocercous type – forked tail
Gymnocephalous type – straight tail
Microcercous type – short tail
Furcocercous
cercaria
Microcercous
cercaria

17. Cercariae may: (1) penetrate the definitive host (schistosomes)
(2) penetrate a second intermediate host and encyst as metacercariae, or
(3) attach to aquatic vegetation and encyst as metacercariae.

18. Metacercaria
METACERCARIA is formed as a cercaria
loses its tail and encloses itself in a
_______________________________
_
Cyst is formed by:
3.
2.
Metacercaria is an
______________________________
Reproductive organs may begin to form but
eggs are never produced.

19. Metacercaria
Metacercariae may occur within a second
intermediate host or on aquatic vegetation.
They are viable for weeks to years.
When the second intermediate host or
aquatic vegetation containing metacercariae
is eaten by a definitive host, the metacercaria
_____________________________in the
digestive tract and matures to the
____________________________.