PARASITOLOGY

larvae develop into. (Metacestode is the term now most commonly used but there are
some dissenters who claim that the use of the term larva is adequate and that to describe
the developmental stages as metacestodes is unnecessary). Where more than one inter-
mediate host is part of the life-cycle, the stage that lives in the second intermediate host
is also called a metacestode.
Metacestodes occur in a number of different morphological types (see Fig. 3.6):

n ProcercoidThe eggs are swallowed mainly by crustacea and the larva hatches out and

develops into a solid spindle-shaped structure with hooks on its posterior. This is typ-
ical of the pseudophyllideans and trypanorhynchids whose definitive hosts have some
association with water.
n PlerocercoidThis stage occurs in a second intermediate host and is a solid structure with

scolex but does not have any hooks. Commonly found among the pseudophyllideans,
trypanorhycans, tetraphyllideans and a few taenioids.
n CysticercoidThis a metacestode that nearly always develops in an insect, in particular

one of the grain beetles (Tenebriospp). It consists of a round anterior cyst-like vesicle

within which is a non-invaginated scolex. There is a ‘tail’ arising from the posterior

region which has hooks.

PLATYHELMINTHS

Protoscolex Germinal membrane

Tetrathyridium

Procercoid

Plerocercoid

Tail

Cysticercoid

Scolex

1st metacestode stage

Hydatid cyst

Brood capsule

Cysticercus

Invaginated scolex

Fluid-filled bladder

Hatched oncosphere

Strobilocercus

Egg membrane

Taeniad egg

Hexacanth larva

• Figure 3.6The larva
  hatches out of the egg and
  develops rapidly into a
  metacestode. Although
  throughout the cestodes
  there is very little variation
  in egg morphology, the
  development and
  morphology of the
  metacestodes differs in
  each individual cestode
  genus. Each type of
  metacestode occupies a
  characteristic niche within
  the intermediate host.