SRG General Parasitology

The Larval Cestodes

1) The Cestodarians

The cestodarians larvae, or lycophore are free swimming, being covered in cillia. They have a set of ten hooks at the extreme anterior of the body, thus differing from the larval eucestodes, which are equiped with 3 pairs of hooks. Anteriorly they are armed with penetration glands. The bodily form of these larvae bears a marked resemblance to the larvae of the trematodes, such as the miracidium in the digeneans, and the larval monogenean, the oncomiracidium.

2) the Eucestodes (Tapeworms)

The eggs of Pseudophyllidean and Cyclophyllidean cestodes differ considerably.


The egg of the pseudophyllidean tapeworm closely resembles that of the trematodes, having a thin shell wall, and an operculum, which on hatching opens to reseales the free swimming larvae. This illustrates the close relationship between the two major groups of platyhelminth parasites,	The larvae emerging from these eggs also differ. The pseudophyllidean egg hatches to release a free swimming larvae called a coracidium. This has an outer layer of ciliated epidermal cells with which it swimms through the water before being ingested by the parasites 1st intermediate host. This is often a copepod. Inside the copepod the ciliated epidermis is shed, to release a larvae that initially resembles that of the newly hatched cyclophyllideans. This has 6 hooks, arranged in pairs, and is a common feature throughout the eucestodes. On the basis of the presence of these hooks, present in both the eucestodes and cestodarians, many authors believe that the cestodes originally evolved from an ancestor common to the extant monogeneans.	The larval cyclophyllidean, as with the pseudophyllidean, is equiped with 3 pairs of hooks. Both groups use these hooks to penetrate the gut wall of its intermediate host after being ingested, before developing into the other larval forms described below in more detail

In contrast, the egg of the cyclophyllideans tapeworms is very different, having a very thick, resistant egg shell, with no operculum.

Metacestodes

A number of different larval forms of cestodes (metacestodes) are seen, these include the following;

	A larval form of Pseudophyllidean cestodes, (e.g. D. latum, Ligula intestinalis). Here two forms of the procercoid are shown. Firstly an immature procercoid, and secondly a mature infective procercoid. In the lifecycle of these parasites there are two intermediate hosts (seebelow). The procercoid being found in the first of these (usually a small crustacean e.g. Cyclops).In appearance these larvae have solid bodies with the remains of the embryonic hooks from the onchosphere larvae at the posterior of the parasite.
	A larval form of Pseudophyllidean and other Cestodes, (e.g. D. latum, Ligula intestinalis). In the lifecycle of these parasites there are two intermediate hosts (see the cestode lifecycle page). The plerocercoid being found in the second of these (usually a fish or amphibian). In appearance these are elongated larvae with solid bodies which are much larger than the preceding procercoid larvae. In these stages the embryonic hooks are absent. The plerocercoids of some Pseudophyllideans already show the start of the development of the sexual organs (e.g. Schistocephalus solidus, Ligula intestinalis), whilst those of Schistocephalus solidus are also already divided into proglottids).
	A larval form of Cyclophyllidean Cestodes, (e.g. Hymenolepis sp.). This larval form is usually found in species where the intermediate host is an invertebrate, usually an insect.
	A larval form of Cyclophyllidean Cestodes, (e.g. Taenia solium)
	A larval form of Cyclophyllidean Cestodes, (e.g. Mesocestoides sp. ).
	A larval form of Cyclophyllidean Cestodes, (e.g. Taenia taeniaeformis ).
	A larval form of Cyclophyllidean Cestodes, (e.g. Taenia multiceps ).
	A larval form of Cyclophyllidean Cestodes, (e.g. Echinococcus granulosus). - Hydiatid cyst

Reproductive System