MET'AZO'A (Nen-Lat. nom. pl.. front Gk.
meta, after ,t5on, animal). The name applied to all the above the Proto zoa. The animal kingdom is thus subdivided into twit divisions, namely. the Protozoa, or one-celled animals, and the Aletazoa, or many-eelled ani mals. The latter include all the branches or phyla of the animal kingdom from the sponges Porifera) to the Verb-brat a. Each metazoan, however, develops from a single cell, the egg.
The IMetazon have been defined as "Animals in which the ordinary (so-called adult) form of the species has always mire than one nucleus, and in which the nuclei are for the most part ar ranged regularly and with a definite relation to the functional tissues of the animal (so-called 'cellular arraug,ement'). Special vonjugating in dividuals of the form of ova and spermatozoa are always formed." Metazoa reproduce by ova and spermatozoa. These' reproduetive products originate by a process of n»equal fission from their parent, and may both be produced by one or different individuals. When they are both
produced by the same individual, that individual is said to be hermaphrodite. When they are pro dneed by different individuals, that parent giv ing rise to the egg is called female, and that producing sperm cells or spermatozoa is called the male, and the individuals are said to be 'uni sexual' and the species `dioselonS.' in certain forms, probably under given conditions of food or temperature, the ova may develop without being fertilized by a sperm cell, the process being called 'parthenogenesis' (q.v.). Reproduction by ova and spermatozoa is called 'sexual reprodue lion,' and that by parthenogenesis 'asexual re prodnetion.' Consult Bourne, The Orlomate Metana (London and New York, 1901). See