te and fusing its nucleus with that of a neighbour of the opposite sex, a
             process that is called conjugation. In ciliate protozoans (e.g., Paramecium), the conjugation
             process involves the exchange of haploid nuclei; each partner acquires a new nuclear
             apparatus, half of which is genetically derived from its mate. The parent cells separate and
             subsequently reproduce by binary fission. Sexuality is present even in primitive bacteria, in
             which parts of the chromosome of one cell can be transferred to another during mating.
             Multicellular organisms also reproduce asexually and sexually; asexual, or vegetative,
             reproduction can take a great variety of forms. Many multicellular lower plants give off asexual
             spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or
             multinucleate. In some cases the reproductive body is multicellular, as in the soredia of lichens
             and the gemmae of liverworts. Frequently, whole fragments of the vegetative part of the
             organism can bud off and begin a new individual, a phenomenon that is found in most plant
             groups. In many cases a spreading rhizoid (rootlike filament) or, in higher plants, a rhizome
             (underground stem) gives off new sprouts. Sometimes other parts of the plant have the
             capacity to form new individuals; for instance, buds of potentially new plants may form in the
             leaves; even some shoots that bend over and touch the ground can give rise to new plants at
             Among animals, many invertebrates are equally well endowed with means of asexual
             reproduction. Numerous species of sponges produce gemmules, masses of cells enclosed in
             resistant cases, that can become new sponges. There are many examples of budding among
             coelenterates, the best known of which occurs in freshwater Hydra. In some species of
             flatworms, the individual worm can duplicate by pinching in two, each half then regenerating the
             missing half; this is a large...