Mechanisms of karyotype evolution in the Monocelididae (Turbellaria Proseriata). Within the Proseriata (Turbellaria: Seriata), the family Monocelididae displays several interesting patterns of chromosomal evolution. A basic set of n = 3 -made up or one large metacentric, one medium-sized metacentric and one small clearly heterobrachial chromosome -has been detected. It is widely distributed in species pertaining to both the subfamilies in which the Monocelididae are subdivided. From this basic set, other complements -either with n = 3 or n > 3 -would be originated.Several karyological mechanisms or common occurrence within the family (translocations, small chromosome rearrangements involving the centromere, genome growth) have been detected. As to the chromosome complements with n > 3 -occurring within the subfamily Monocelidinae -they can be interpreted as the result of a fission of one or both the metacentric chromosomes of the basic set. Karyometrical data are in good agreement with this suggestion. Centric fusion or polyploidy, well known for fresh- water Triclads, have been never found in the family
