caecum may have many smaller branches extending out of it. The inner surface of
the gut, the gastrodermis, both absorbs food and secretes digestive enzymes. All the
organelles normally associated with active and passive absorption of digested food are
present in the cells of the gastrodermis.
The outer coveringof trematodes is a non-ciliated tegument which like that of the
cestodes can absorb soluble nutrients and is also immunogenic. In structure and func-
tion the trematode tegument is very similar to that of the cestodes. The adult schisto-
somes live in blood vessels and an apparent adaptation to that environment is that
the tegument (originally a trilaminate outer membrane) has become a heptalaminate
membrane.
The excretory systemis composed of numerous flame cells that empty into collecting tubules.
The flame cells are also present in the cercaria and their number and arrangement vary
with species.
The reproductive systemis normally hermaphrodite, with two exceptions where the
adults are unisexual: the Schistosomatidae and Didymozoidae.n The male reproductive organs develop first (protandry). Two testes is the norm and
two vasa efferentialead from them into a single vas deferensending in a cirrus opening
into a genital sac. Surrounding the cirrus is a ductus ejaculatorisand a seminal vesicle.
The pattern of spematogenesis is that 64 spermatozoa arise from a sperm bundle (a pat-
tern common to most platyhelminths).
n The female has a single ovary and oviduct. Two vitelline ducts lead from the lateral
vitelline glands into the oviduct which forms a chamber, the ootype (where the eggs
are formed). Surrounding this area is a complex of glands known as Mehlis glands. A
canal — Laurer’s canal — leads from the oviduct and opens out on the dorsal surface.
This structure is thought to be homologous with the vagina of the Cestoda.The eggs are covered by a protective shell. In most of the digenea, monogenea and
the pseudophyllidean cestodes the egg shell is made up of a ‘tanned’ protein sclerotin.
The egg shell precursors have their origin in the vitelline glands (the vitellaria).
Histochemical staining of the vitellaria shows that they contain the component proteins,
phenols and polyphenol oxidase of the quinone-tanning system. Various functions have
been described for Mehlis’ gland and they include: lubrication for the passage of eggs;
activation of spermatozoa; release of shell globules from the vitelline cells; activation of
the quinone-tanning process; and creating a membrane to serve as a template on which
shell droplets accumulate to form the egg-shell.n The majority of eggs are non-embryonated and pass out into the environment via the
faeces or urine, or in the case of lung infections via the sputum. Aquatic or moist con-
ditions are required for the eggs to embryonate and develop.
n If the temperature, light, osmotic concentration and oxygen concentration are optimal,
a motile miracidium hatches out into the water. The miracidium then swims around
until it encounters the mollusc intermediate host. The eggs of Dricocoelium dendriticum
hatch only when ingested by the intermediate host.The miracidiumThe outer layer of the miracidium is covered with cilia. At the an-
terior end are three glands, two outer penetration glands and in the middle the apical
gland. Below the glands there is a collection of nerve cells linked to two lateral nerves.
Flame cells are scattered through the interior. There are two main types of cell: thePARASITOLOGY