PARASITOLOGY

n Aspidogastrea are endoparasites with the entire ventral surface forming an adhesive organ.
n Monogenea are ecto-parasites of poikilotherms and have direct life-histories, ie

involving only one host eg Polystommaspp. (The larval parasite is found in the gills of
tadpoles and the adult migrates to the bladder of the frog.)
n Digenea are mainly endoparasites with one or two simple attachment organs and

more than one host in the life-cycle. The intermediate host (apart from two excep-

tions) is always a mollusc eg Fasciola hepatica; Schistosoma mansoni; Clonorchis sinensis;

Paragonimus westermania.

The adult digenean body consists of a single non-segmented oval or leaflike dorsovent-
rally flattened structure (see Fig. 3.7). The attachment organs are an oral sucker and a
ventral sucker (also known as an acetabulum).
Adult digenea with rare exceptions occur exclusively within vertebrate hosts. The
most common habitats are the gut, the lungs, the bile duct. One family, the
Schistosomatidae, nearly all live in blood vessels. Other less common sites are the swim
bladder, the eye and the urino-genital system.

n 3.10.1 THE BODY STRUCTURE OF TREMATODES
All trematodes have a mouth, a muscular pharynx and a well developed alimentary
canal, suitable for the digestion of semi-solid or viscous foods such as blood, bile fluid,
mucous and intestinal contents.
The alimentary canalin digenea is a bifurcate structure. The mouth opens into the
pharynx and this leads into an oesophagus and then into two blind-ending caeca. Each

PLATYHELMINTHS

Cirrus pouch

Cirrus

Ventral

sucker

Ovary

Vitelline

duct

Vitelline

gland

Testes

Mehlis gland

Vas deferens

Uterus

Intestine

Pharynx

Oral sucker •Figure 3.7An adult

trematode has a single

flattened non-segmented

structure and, apart from

the Schistosomatidae, is

hermaphrodite. They have

an alimentary canal with

a single opening

leading into pharynx

and gut diverticula. The

reproductive organs are

embedded in a layer of

parenchyma tissue.