Life Cycles of Digenetic Trematodes

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? 

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.

Generalized Life Cycle

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.

Egg = OvumAfter 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?

Egg = Ovum2. ________________________________________- 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

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 ______________________________

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:

MiracidiumMorphology: 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.

MiracidiumUpon entering a snail, the miracidium loses its cilia and transforms into a sac-like body called the __________________________________________

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?

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 ______________________________

Redia

Mother rediae burst out of the mother sporocyst and migrate to the __________________________________________________________of the snail host.

hepatopancreas

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 _____________________________.

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

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

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.

Metacercaria

METACERCARIA is formed as a cercaria loses its tail and encloses itself in a ________________________________

Cyst is formed by:

1.

2.

Metacercaria is an ______________________________

Reproductive organs may begin to form but eggs are never produced.

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 ____________________________.