Turbellarian taxonomic database

Abstract / Notes

In various spp. of triclad Turbellaria there are certain modifications of the genitalia which can best be
interpreted as mechanisms for the prevention of self-fertilization. In spp. of Planaria the size and
musculature of the penis and its manner of insertion are sufficient to prevent self-fertilization, but in
other genera, notably Dendrocoelum and Bdellocephala, the penis has been reduced and self-fertilization is
prevented by other modifications. In Dendrocoelum the reduction of the penis is accompanied by the
development of an introversible extension (the flagellum) at its end. When this flagellum is in the
introverted position, which is the position of rest, it acts as a valve and prevents the escape of sperm.
When extended during copulation, it assists by increasing the effective length of the penis in penetrating
into the "uterus" of the co-copulant, and prevents the escape of sperm into its own atrial cavity. In
Bdellocephala the penis is so reduced as to be no longer functional as such. The transfer of sperm is brought
about by the muscular gland organ which has an extensible papilla which is inserted into the partner during
copulation and which withdraws the spermatozoa from the cup-like penis. The possibility of self-fertilization
occurring on the withdrawal of the muscular papilla is prevented by the extension of a flap from the dorsal
wall of the genital atrium, which puts the [male] atrium into direct communication with the exterior. Gelei
(1924) suggested that self-fertilization did not occur because of physiological antipathy between the two
sexual elements of the same animal. This is unlikely, in view of the mechanisms mentioned above.

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