A phylum comprising about 100 species of simple nonvascular plants, the hornworts (or horned liverworts), found worldwide in temperate and tropical regions on tree trunks, riverbanks, and other damp locations. They resemble thallose liverworts (see hepatophyta), but produce long horn-shaped green sporophytes, which split longitudinally to release the spores. Hornwort cells each contain a single chloroplast inside which, uniquely among plants, is a pyrenoid, associated with starch production. Some species have separate male and female plants, while others have both types of sexual organ on the same plant. The motile sperm swim through the surface water film to fertilize the female gametes, and the resultant embryo gives rise to young sporophytes. Young gametophytes arise directly from the germinating spores.

Hornworts were formerly classified as a class (Anthocerotae) of the Bryophyta.

http://www.ucmp.berkeley.edu/plants/anthocerotophyta.html An entry into the world of hornworts, with links to images