3.8.1 Prominent features
n The adult has an elongated tape-like body divided into proglottids.
n The absence of a gut in both the larval (metacestode) and adult stages.
n The outer covering is a living layer known as a tegument. All nutrients are absorbed
through the metabolically active tegument, often referred to as naked cytoplasmic
layer. It has the same structure in both the adult and metacestode stages.
n Apart from one group, Dioecocestusspp, all cestodes are hermaphrodites. They are
monoecious ie they have both male and female organs and are protrandrous (males mature
before the females).3.8.2 Cestodaria (A relatively primitive sub-class)
The body is not divided into proglottids.n Only one set of reproductive organs.
n No scolex; decacanth larva.
n eg Amphilina foliacea, whose definitive host is Acipenser(a sturgeon) and the intermedi-
ate host is Gammarus(a shrimp).3.8.3 Eucestoda (The main sub-class of tapeworms)
There are four common orders which are universally recognised: (1) Tetraphyllidea;
(2) Trypanorhynca; (3) Pseudophyllidea; and (4) Cyclophyllidea.
The Eucestoda are divided into proglottids (except for one group, the Cario-
phyllidea). Each mature proglottid contains a complete set of reproductive organs.
The eggs produced after fertilisation either embryonate in the uterus (known as an
oncosphere larva) or in water (a coracidium larva). Both larvae have six hooks (hexacanth
larvae).3.8.3.1 The egg
The eggs are enclosed in a capsule or egg shell divided into an outer envelope, an inner
envelope and an oncospheral membrane. There are two main groups of eggs: group 1
have an aquatic stage; and group 2 have no aquatic stage.
Group 1 eggs are laid in water and pass into an aquatic intermediate host. Most of the
eucestodes except the Cyclophyllidea belong to this group. The Pseudophyllidea have an
egg capsule with a ‘lid’ (operculate).
Group 2 eggs are embryonated and are laid normally onto soil and very rarely in
water. The capsule when present is sometimes thin. The inner envelope is made of two
zones: The outer zone (Zone I) is a cytoplasmic layer and the inner zone (Zone II) is a
gelatinous layer. The innermost part of Zone II is the embrypohore.3.8.3.2 The scolex
The anterior region, the scolex (the ‘head’ region), has attachment organs and is embed-
ded into the gut mucosa (see Fig. 3.1). There are three types of attachment organs:
bothria, bothridia and acetabula.
Bothria are narrow grooves with a limited amount of muscle; and they form suck-
ing organs. Bothridia are mobile, stalked or sessile attachment organs and are mainly
broad, flattened structures with thin flexible margins. They are mostly found in the
Tetraphyllidea and Pseudophyllidea. Acetabula are sucking organs typically found in thePARASITOLOGY