Since the discovery of sporocysts and
rediae "by Steenstrup in 1842, the study of the life
histories of different Trematodes has received much
attention by investigators, for instance Wagener in
1857, leuckart in 1882, Thomas in 1883, Heckert in
1889, Greutzberg in 1890 and Looss in recent times.
It was from the observations of these workers that
the life cycle of Trematodes was put on a firm basis.
But no real importance was attached to the study of
miracidia, sporocysts, rediaeand cercariae until after the classical work of Thomas in 1883, which resulted in a thorough explanation of the life history
of the liver fluke and its transmission from sheep to
sheep.Since 1883 numerous investigations have
been carried out from the economical and medical
point of view, and the bulk of the contributions to
the knowledge of cerca.riae consists of the results
of observations and experiments on the cercariee of
the Trematodes infecting man and the domestic animals Particular attention was paid to the different intermediate hosts and the conditions under which they are
capable of causing the infection of the definitive
hosts.A serious study of the organisation of cercariae
was long neglected and in spite of the numerous cercariae described by many investigators, it is
impossible to identify many of the species correctly.
It was only after the study of the cercariae of the
Schistosomidae infecting man that the study of the
morphology and histology of other cercariae was adequately undertaken, and authors began to attach importance to features of the internal organisation, which
may throw light on the structure of the adult. The
systems which have been carefully studied are the alimentary tract, the excretory and genital systems, and
specific differences were observed even among cercariae closely resembling one another. Certain
glands in the body have also been observed, and have
proved in many cases to play an important part in the
process of infection of the definitive host.If the external and internal structures of
a cercaria be considered, we may divide the characters into two sets:(A) 'Those structures such as the suckers,
the alimentary tract, the excretorysystem,
the genital system, and some
parenchymatous cells which form the
basis or are the miniatures of the
corresponding adult structures,(B) Those structures such as tails, spines,
processes, stylets, eye spots, salivary glands, cystogenous cells and
parenchymatous cells,which are purely
larval structures and are not carried
over to the adult. The structures
belonging to the former group are relatively well known but in the case
of the larval structures there is
much yet to be explained.What are termed salivary glands may have the
same structure but different functions in different
cases. The cystogenous cells are distinguishable
from parenchymatous cells by possessing larger nuclei
poor in chromatin, and cytoplasm which may contain obvious granules or may be faintly granular, but their
function is not always demonstrable. Lastly the
parenchymatous cells are indistinguishable from other
cells such as those surrounding the nerve ganglia,
those under the cuticle, which have received the
name of cuticular cells and from cells which are destined to give rise to other structures such as vitelline glands. Looss emphasized the fact that thesecells are at first indistinguishable cytologically
and maintained that many of these mesodermal cells
remain findifferentiated for a considerable time.
There is also reason to believe that some cells in
the parenchyma form the basis of other structures
such as the excretory system, for the collecting
ducts are not bounded by cellular walls, but are
merely lumina among certain cells. Certain other
parenchymatous cells both in the body and in the
tail of some cerc&riae are more vacuolated and distended and their function appears to be that of
buoyancy
