Introduction to the Platyhelminthes
Brief Taxonomy of Platyhelminthes
Basic Body Plan
The Tegument
Reproductive System
Digestive System
General Lifecycle
Platyhelminthes - The Generalised Reproductive System
The reproductive systems of these organisms shows a considerable amount of variation, but there are common features present in many of the main groups, particularly the parasitic platyhelminthes. A detailed description of the reproductive systems found in the turbellarians is beyond the scope of these pages, but a few brief details will be of interest.
The most primitive turbellarians, such as those belonging to the Acoela, have no discrete gonads, and reproduce by asexual fission, (a common form of reproduction carried out by many other turbellarians, even those with fully developed sexual systems). In this case the fission is transverse, in some cases, particularly in the more primitive groups, the individuals remaining attached to each other, to form chains, (the individuals comprising these chains being referred to as zooids. In some of the freshwater and terrestrial planarians fragmentation rather than fission may occur, each fragment forming a cyst from which eventually a small worm emerges.
The most developed sexual reproductive systems within this group are found in groups such as the planarians, which show striking similarities with those of the parasitic groups. In the female portion of the system there are, in the planarians often only a pair of ovaries as illustrated here (labelled "O") located at the anterior end of the worm, close behind the eyes. Here the eggs are produced. In the platyhelminthes the eggs are unusual in that the yolk material is usually separate from the cytoplasm of the egg, but is situated in yolk cells that surround the egg as it is released from the ovary. These yolk cells are usually produced in numerous yolk ("yg"), or as they are more commonly called, vitelline glands, connected by ducts to the oviduct (NB in some groups the ovaries and vitelline glands may be united). In other turbellarians there may be numerous pairs of ovaries, but only ever a single pair of oviducts ("Ov"), which extend backwards along the lateral margins of the organism, finally opening into the genital chamber ("Gc"), the analogous to the genital atrium found in the parasitic groups.
The male reproductive system consists of the testes ("T"), which in the planarians may be numerous, connected by sperm ducts (the vas deferens, "vd") to a penis ("P"), often armed with a stylet, which protrudes into the genital chamber. In many of the turbellarians, particularly the polyclads, there may be multiple male organs, including multiple penis bulbs and stylets, present. These are, at least in some cases, not used for reproduction, but as defensive organs. In some cases the penis is replaced by a eversible structure called a cirrus.
In the planarians, although they are hermaphroditic, self fertilisation is not carried out, although this form of reproduction does occur in other groups of platyhelminthes, particularly the parasitic groups. In the planarians the penis is protruded out of the genital pore ("Gp"), and is inserted into the copulatory sac ("Cs") of its partner, where sperm is deposited. Usually copulation is reciprocal, both partners fertilising each other. After the planarians separate, the sperm, which characteristically of the whole phyla are biflagellate, leave the copulatory sac, swimming up the oviduct to fertilise eggs as they are discharged from the ovaries. The fertilised eggs then pass down the oviducts, and the egg begins to form. Finally the yolk cells then surround the eggs as they move down the oviduct before passing out of the genital pore.
For the parasitic groups, the Monogeneans, Digeneans, Aspidogastreans and Cestodes, the reproductive organisation is very similar. The details of these systems, as related to the Digeneans and Cestodes, are given in separate pages within this site, (select these links to go to the Cestode or Digenean pages). In this page a simple overview and comparison between the different groups is given.
In most cases, these organisms are hermaphroditic, an important exception to this being with the schistosomes. The male system consists of a single testis in the Aspidogastreans, usually a pair of testes ("T")in the Digeneans and a variable number in the cestodes, the position and shape of which are of considerable taxonomic importance. From the testes sperm ducts (the vas deferens, "vd") carry sperm to the cirrus sac("Cs"). This structure contains the seminal vesicle ("sv"), prostate gland ("pr") and cirrus ("c"), or, in some cases, penis. This cirrus sac opens into a common genital atrium ("CGA"), shared with the female uterus, in the digeneans, or vagina in the cestodes. On leaving the testes sperm are stored in the seminal vesicle, before being released during either copulation, when the cirrus or penis is placed in the uterine or vaginal opening of its partner, or during self-fertilisation, when it is inserted into its own female opening, this being a common occurrence in these parasitic groups.
The central region of the female system is the structure characteristic of these parasitic platyhelminthes, the Öotype ("Oo"). This receives eggs from a single ovary, which is usually bi-lobed in the cestodes, as illustrated here, ("O"), the yolk cells from vitelline glands (which are usually numerous), and sperm, which after entry into the female system are often stored in a seminal receptacle.
Surrounding the öotype is a glandular region, the Mehlis gland ("M"). The function of this gland is unclear, but may help nourish the developing egg, and provide lubricating fluid, aiding the movement of the eggs in the öotype and uterus. Sperm may enter the female reproductive system in a number of ways. In the Trematodes the vagina ("v") is replaced by a specialised copulatory canal, "Laurer's" canal ("l"), which in the Digeneans opens from the dorsal surface of the worm, and in the Aspidogastreans ends in a blind ended sac. Laurer's canal may be used by the flukes during copulation, but more normally sperm enters the female system via the common genital atrium, (into which the uterus, ("Ut") open), either during copulation, or self-fertilisation. In contrast, in the Cestodes it is the vagina that opens into the common genital atrium, and sperm enter here, usually by self-fertilisation. The Monogeneans have a similar female reproductive system to the digeneans, except in some cases there may be two vaginas present. The ova released from the ovary reaches the ootype via the oviduct, where it is fertilised by the sperm, and is then surrounded by yolk cells and material that hardens around the egg ("E"), to form an egg shell. Both the yolk cells and egg shell material are derived from the vitelline glands ("vt"), which are connected to the öotype via the vitelline duct, ("vt.d"). The eggs then enter the uterus, usually leaving the parasite as they pass out of the genital pore. An important exception to this occurs in the Cyclophyllidean cestodes. In these the uterus does not form an opening, but is a sac-like structure within which eggs accumulate.