Reproductive biology

Asexual reproduction is a common method of reproduction in freshwater and terrestrial turbellarians. Many of the triclads divide by transverse fission: the body splits transversely behind the pharynx and each part generates the rest of the body. The posterior portion attaches to the substrate and the anterior portion crawls away until it tears in two. In species such as Dugesia, the cells tend to vary in their ability to regenerate. The cells in the middle portion of the body have the strongest ability to regenerate. Experiments have shown that if just the tail is cut off, it will not grow a new body, whereas the main portion of the body will regenerate a new tail. The ability of the fissioned portions of these worms to regenerate the proper half has interested scientists for years in investigating why the head portion grows a tail and why the tail portion regenerates a new head. In the genera Catenula, Microstomum, and Stenostomum (orders Catenulida and Macrostomida), multiple transverse planes develop that lead to a train of individuals called zooids that do not detach until they reach a certain stage. Other species (e.g., Phagocata and some terrestrial species) detach fragments that become encysted and eventually develop into new individuals.

Turbellarians are hermaphroditic and their sexual reproductive systems are quite complicated. The male system may have one, two, or multiple testes that drain via sperm ducts that may lead to a storage area called the seminal vesicle. Prostate glands may be present that produce seminal fluid that mixes with the sperm in the seminal vesicle. The sperm then exits the worm via the protrusible penis or eversible cirrus with help from a muscular ejaculatory duct. The female system is more variable among the Turbellaria, depending on whether they produce entolecithal (Macrostomida and Poly-

cladida) or ectolecithal (Rhabdocoela, Prolecithophora, and Tricladida) ova. A germovitellarium, which may be single or paired, produces entolecithal ova (yolk reserves within ova). A germarium or ovary, which is separate from the vitellaria, produces yolk-free ova that eventually are surrounded by separate yolk cells in a tanned protein capsule to form the ec-tolecithal egg. Sperm also are included within the egg capsule to insure fertilization. Eggs pass through the oviduct that may be differentiated into a seminal receptacle or uterus before deposition.

Cross fertilization (mating) usually occurs when worms align themselves with each other, and the cirrus or penis of each worm is inserted into the female gonopore or atrium of the other and deposits sperm. The worms then go their own way with the sperm stored in their seminal receptacles. In some species, mating occurs by hypodermic impregnation in which the male copulatory organ penetrates the body wall of the mate and deposits sperm in the mesenchyme. The sperm then make their way to the ova.

Turbellarians have either direct development or produce a pelagic larva. Polyclads often produce a pelagic Muller's larva that settles to the bottom and goes through metamorphosis in a few days. This larva has eight ventrally directed ciliated lobes, which it uses to swim. Stylochus, a parasitic poly-clad, produces the Gotte's larva, which has only four ciliated lobes.

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