Unit 3: Helminthes (Cestodes)
54
Lecture 1 - Introduction
Phylum Platyhelminthes
Class Cestoidea
General characters
The cestodes or tape worm are parasitic in all or nearly all
stages of the life cycle.
Adult worm live attach to mucosa of the small intestine of
vertebrates
The larval stage is parasitic in the tissue or body cavity of
vertebrate or invertebrate host.
All adult worm of man consist of the following parts:
Scolex (head) or hold fast organ.
A neck immediately behind the Scolex (a region of
growth and proliferation).
A strand of proglottids or segment
Immature ---- mature ----- gravid contain mature eggs.
Strobila is the entire series of proglottids and neck.
Number of proglottids varies from 3 or 4 segment in
Echinococcus species to 1,ooo or more in beef tape
worm and 3,ooo or 4,ooo in the fish tape worm .
The worm is flattened dorsoventrally, creamy to
chalky white in colour, covered with transparent
tegument,which is a syncytium with an outer covering
of microvilli (microtrichs)
Scolex: (head) knob like provided by 4 cupped suckers
2 ventrolaterally, 2 dorso laterally
Or the Scolex is spatulate provided with elongated median
vental and dorsal sucking groove
Excretory system: The major dorsal and vental
longitudinal excretory tubules run along the lateral margin
of the segment.These joined by anastomoses in the Scolex
and by transverse anastomoses near the posterior margin
of each segment which collects the waste products
through the terminal flame cells and numerous tubules.
Nervous system
:
consist of several ganglia with
commissure in the Scolex. Lateral longitudinal nerve
trunks connected by Transverse commissure.
Digestive system: Cestodes lack digestive organ
Tape worms that infect human, belong to 2 orders
Pseudophyllidea e.g Diphyllobothrium
Cyclophyllidea e.g Taenia ,Echinococcus,
Hymenolepis
Genital organ: Tape worm is a hermaphrodite. There is
one complete set of male and female organs for each
proglottids. The genital pores may be located mid lateral
(Taendia) or mid ventral in the anterior half of the
proglottids (Diphyllobothrium) in which the uterus is
provided with a pore through which unembryonated egg
are discharged.
In order Pseudophyllidea, the eggs are operculated and
unembryonated when laid.
In order Cyclophyllidea the eggs have two layers and
contain mature embryo, which contain 6 hooklets called
an Oncosphere.
Developmental stages of Tape worms
Eggs are passed in faeces or gravid proglottids become
detached and pass out of the bowel
Order Pseudophyllidea
: Solid form larvae
The oncosphere undergo development inside the egg in
fresh water and in one to two weeks a ciliated embryo
called Coracidium
hatches through the operculum
consists of typical hexacanth embryo surrounded by
ciliated membrane.
In the first intermediate host (Cyclops) the hexacanth
embryo develops into an elongated worm like organism
with large knob like structure (cercomer) on the posterior
end , which contains the emberyonic hooks and a slight
depression is formed at the anterior end in which lytic
glands develop.This larval form known as Procercoid
about o.5 mm in length.
In the Plankton
_
eating fish (2 intermediate hosts) the
procercoid develop into (Plerocercoid) which is
elongated flattened, wrinkly white mass of tissue with
inverted Scolex at anterior end.
Unit 3: Helminthes (Cestodes)
55
In order Cyclophyllidea
The larval stage is bladder
or cyctic, vesicular form.
Cycticercus
:
refered to as a bladder worm. is a small
bladder filled with fluid and containing one invaginated
Scolex attach to the inner wall.
Cysticercoid
:
is a small solid larva with invaginated
Scolex lies within solid tissue instead of fluid and the body
tapers into a tail like projection.
Coenurus
:
large bladder worm with many invaginated
scolices in the fluid_ filled bladder.
Echinococcus
:
a large bladder or hydatid which internally
produces multiple scolices (protoscolices) and numerous
daughter bladders or brood capsules.