Unit 3: Helminthes (Trematodes)
48
Lecture 1 - Introduction
Class: tremaoda
Subclass: Digenea
Only Digenetic trematoda produce infection in man.
Adult trematodes are parasites of vertebrates also called
Flukes.
Trematades are hermaphrodites or monocious ,but
schistosomes are unisexual(diecious)
Life cycle of Trematodes :
Adult worm liberting fertilized egg in water hatch into
ciliated miracidium (1
st
larval stage) which is infective to
molluscan hos. In the mollusk hemolymph spaces,
transform to elongated sac ,1
st
generation sporocyst
(second larval stage )
Germ cell developed from the inner wall give either to
number of 2
nd
generation sporocyst in some species or to
radia (3
rd
larval stage) in other spp.
Asexual multiplication occurs in this stage & 2
nd
generation of organisms escape from the first & grow &
internally produce cercaria (4
th
larval stage)
The cercaria when mature emerge from the mollusk &
temporarily become free living
According to the nature of the tail ,different name are
given :
1) furcocercus :fork-tailed cercaria
2) Microcercus :short , stumpy ,rudimatory tail
e.g.paragonimus
3) lophocercus : large , floode e.g. clonarchis
4) pleurophocercus : long ,powerful tail & apair of
finfolds e.g.opisthorchis
Metacercaria: a stage of development between the
cercarial & adult stages which is an encysted cercaria
without a tail .so depending on a particular group the free
cercaria:
a) Becomes attached to the skin of the definitive host,
discards it ۥs tail & penetrate he mucous membr.
b) crawls onto an aquatic plant ,drop it ۥs tail ,round up &
encysts by covering itself with material secreated by
cystogenous glands e.g.:fasciola
c) or shed it ۥs tail & penetrate he tissues of aquatic
animal like fish e.g.clonorchis ,Heterophys or crab as
crayfish e.g.paragonimus or terrestrial animal
e.g.Dicrocoelium in which it become encysted. In the
later two types of development, the diffini.host
become infected after ingestion of the encysted
cercaria in uncooked veretable or animal tissues.
The mature worm: vary in size & shape .Some are large
& fleshy ;other are thin & flabby ;other are small but
blood inhabing fluks are delicately cylindrical .
There are 2 sucker :the oral sucker surrounding the moth
opening on the ant.end of the worm & vertral sucker
(acetabulum) (which is a cup shaped ,muscular by which
the worm attach to the host on the veritral surface
Digestive system:
Consist of a mouth then an esophagus that is provided by
with spherical or pyriform ,muscular pharynx then two
intestinal caeca which run parallel & blindly near the
post. End of the body .They may be simple or brunched or
they reunite to form a single tube.
Reproductive system:
All digenetic trematodes except schistosomes are
hermaphroditic
Male Reproducti ve system:
Consist of 2 testes from each one vas defferens arise ,join
to form vas deferens ,pass forward ,a swollen seminal
vesicle , prostate gland , muscular cirrus or penial organ ,
the cirrus sac surround these terminal male genital organ
which open into the commone genital atrium which
provided with genial pore.
Female Reproducti ve system:
Consist of single ovary ,oviduct seminal receptacle ,laurer ۥ
scanal (open on the dorsal surface ) ootype (which is the
chamber where the eggs fertilized)surrounded by Mehlis
gland (associated with egg shell).
Vitelliaria that contain yolk cells & shell gland lie along
the margin of the body & ducts from the gland open into
the ootype .Coiled uterus originate on the ant. face of the
ootype &proceed to the genial atrium.
Eggs are operculated ,except schistosome egg non -
operculated.