are released with mouse faeces and thus dispersal is passive - through the movement and defecation of mice. Adult worm infections in mice held under laboratory conditions persist for 6–12 months. Worms reproduce sexually via the cross fertilization of segments, each of which contains a complete complement of male and female reproductive organs (hermaphroditic). Shelled embryos develop in the ovaries through spiral cleavage, to become infective larvae with 3 pairs of hooks. When ingested by beetles, these larvae use their hooks and secretory glands to penetrate the gut of the beetle and enter the haemocoel where they undergo complete metamorphosis into cysticercoid larvae, replete with an adult scolex, ready for establishment in the final host. Cytology Like all flatworms (phylum Platyhelminthes), H. microstoma maintains totipotent stem cells (called neoblasts in flatworms) throughout its life cycle. These are located in the neck region of the adult worms and are responsible for the continual production of new organs during the process of strobilation (segment formation). Neoblasts divide in the neck region and become incorporated into new segments where they eventually differentiate into the reproductive organs and other elements of the body. The diploid chromosome number of H. microstoma is 12 and the total genome size has been estimated by the Sanger Institute to be 1.4 megabases (with GC-content of ~35%). These values are similar to the genomes of the fox tapeworm Echinococcus multilocularis and the pig tapeworm Taenia solium. All three species belonging to the tapeworm order Cyclophyllidea. Genome sizes outside of this order are presently unknown. Evolution The parasitic flatworms, which includes tapeworms, flukes and monogeneans, evolved from a single major lineage of free-living flatworm ancestors. The switch from a free-living to a parasitic lifestyle in the common ancestor of the parasitic flatworms involved a fundamental change in their tegument, which is found in all contemporary groups. Early-branching tapeworm groups are found in bony (e.g. teleost) and cartilaginous fishes (e.g. sharks and rays) and have entirely aquatic life cycles involving arthropod (e.g. copepods) first intermediate hosts and vertebrate (fish) final hosts. Tetrapod hosts (including mice and humans) were acquired later in tapeworm evolution and eventually part-aquatic life cycles led to the evolution of fully terrestrial life cycles, albeit still involving an arthropod intermediate host and a vertebrate definitive host. Hymenolepis microstoma is a member of the Cyclophyllidea, one of the youngest and most species-rich group of tapeworms. Research impact Most of