The cercaria

Some of the types of Cercariae 

1. Gymnocephalus (e.g. Fasciola sp.)
2. Trichocercous(e.g. Donax sp.)
3. Furcocercous(e.g. Schistosoma sp.)
4. Microcercous(e.g. Bithynia sp.)

  In almost all species of trematode, it is the cercarial stage that emerges from the mollusc, and is the infective form for the vertebrate host, although there may be exceptions to this general rule. For example in some cases a sporocyst, modified to have a thickened internal wall resistant to the environment, emerges, to be ingested by a second intermediate host, (e.g. as is the case in the trematode Dicrocoeloides petiolatum).

  Other exceptions, involving redia producing progenetic metacercaria, have already been described above. The trematode cercaria exhibits considerable variations in structure, which is very important taxonomically, and reflects in many cases adaptations to the specific lifecycle of the parasite involved. Because of this great diversity of form, a system of cercarial classification has evolved, based on the gross morphology of these larval forms. Firstly cercariae may be divided into three major groups;


i) Monostome Cercariae - These lack a ventral sucker, and have simple tails. These forms develop within rediae

ii) Amphistome Cercariae - In these the large ventral sucker is situated at the base
of a slender unbranched tail. These forms develop within rediae.

iii) Distome Cercariae - This is the commonest cercarial form, with the ventralsuckerlying some distance from the posterior end, in roughly the anterior third of the body. These distome cercariae may themselves be divided into a large number of subgroups, based on other morphological features, particularly the form that the cercarial tail takes. Some of these forms are described below;

a) Leptocercous Cercariae - These cercariae have straight slender tails, which are much narrower than the cercarial body. This form is further subdivided into;

i) Gymnocephalous Cercariae - In these, the suckers are equal in size. This is a
common form, represented within such species as Fasciola hepatica, and develop within rediae

ii) Xiphidiocercariae - These are similar to the gymnocephalous forms, but in these the oral sucker is equiped with a stylet, used in penetration of their next hosts, and they generally develop within sporocysts.

 iii) Echinostome Cercariae - In these there is a ring of spines at the anterior end of the larvae, as in adult forms of these parasites. These are found within trematodes of the genus Echinostoma, and develop within rediae.

b) Trichocercous Cercariae - These forms have long tails, equiped with rings of fine bristles. They are usually found in marine trematodes.

c) Cystocercous Cercariae - In these the end of the tail is highly enlarged, with a cavity into which the larval body may be retracted. These usually develop within sporocysts.

d) Microcercous Cercariae - Cercaria with vestigial tails, and which may develop within both rediae and sporocysts.

e) Cercariaea Cercariae - Cercaria with no tails, where the cercaria is not a free swimming form, and may develop within both rediae and sporocysts.

f) Furcocercous Cercariae - In these the tails are forked at the end. The cercaria of the most important group of trematodes, the schistosomes, have cercariae of this form. This form develops within sporocysts.


Otherwise, both externally and internally the structure of the body of the cercaria resembles that of the adult trematode into which they grow. For example, the ring of spines found at the anterior end of echinostome cercariae is also present in the adult flukes.

    The outer surface of the cercaria is a tegument, which may however differ from that found in the adult form in a number of ways. For example in the schistosomes the tegument is covered with a trilaminate plasma membrane, (as opposed to the two bi-lipid membranes found in the adult), on the outer surface of which there is a glycocalyx, (absent in the adult).

  However, many other features of this tegument appear similar to that of the adult, the differences almost certainly being adaptations due to the differing environments that these two lifecycle stages experience. For example, spines found on the surface of both forms of tegument, and the overall structures of a syncytium conected to subtegumental cells are the same.

  Within the cercarial body a number of different types of gland cells may be found, including cystogenous gland cells, used by the larvae to secrete a cyst wall during the formation of the metacercarial stage, and penetration gland cells, used by the cercaria to penetrate its next host, either a second intermediate host, or in some groups the definitive host, (such as the schistosomes), where the cercaria is the final larval stage.

   The cercariae released from their molluscan intermediate host are usually a free
swimming form. These must then locate either their next, and usually final
intermediate host, their definitive host which they actively penetrate (e.g. in  members of the family Schistosomatidae), or locate a suitable solid substrate to encyst upon, or be ingested by their definitive host (members of the family Azygiidae).


   To locate these various targets the cercariae are equipped with a variety of sensory organs. These commonly include two or more eye spots, as well as touch receptors, and allow specialised cercarial behaviour, designed to bring the cercariae into an environment giving the maximum probability of infecting their next hosts.

   For example, the cercariae of the schistosomes exhibit negative phototrophy (swimming to the surface of the water), and positive thermotrophy and thigmotrophy, being attracted to warm objects moving in the water. As well as these behavioural responses within the free swimming cercariae, the parasite exhibits definite circadian rhythms in terms of shedding from the molluscan host, again being shed at times optimal for bringing them into contact with their next host.

    For example, the schistosome cercariae are generally shed during daylight, in the morning, whilst those of other species emerge only at night. In a few groups, such as Alaria spp. However, the parasite employs three intermediate hosts. In these cases the cercaria penetrates the second intermediate host to form a resting stage, the mesocercaria described below. In these cases this
second intermediate host is in turn ingested by a third intermediate host, where it encysts to form a metacercaria.