=Hydroids.=—Belong to a division (Hydrozoa) of the stinging-animals or Coelenterata. They occur both in the sea and in fresh water, and are solely polypoid (i.e. tubular and tentacled). =Hydromedusae.=—Also Hydrozoans. Hydroid colonies with special sexually reproductive persons, which are often liberated as floating bells or discs. =Idio-plasm= (=A and B=).—_See_ pp. 31 and 32. =Ids.=—_See_ p. 123. =Invertebrata.=—Animals with a dorsal heart and without a backbone. =Karyokinesis.=—The changes which are observed in the nucleus both immediately before and after cell division. _See_ Chromatin threads. =Lamarckian factors.= _See_ Somatogenetic characters. _Also_ Neo-Lamarckians. =Metaphyta.=—Multicellular plants (q. v.). =Metazoa.=—Multicellular animals (q. v.). =Micellae= (=Nägeli=).—_See_ Molecules, with which they are identical. =Microaomata.=—The protoplasm of certain vegetable cells is in places characterized by the presence of minute corpuscles, which may be regarded as part of the protoplasm, and are certainly of a protoplasmic nature. These are termed Microsomata. =Molecules= (=Weismann=).—_See_ p. 122. =Multicellular organisms.=—Organisms composed of many cells, as distinguished from the Unicellular organisms, where each individual is constituted of only one cell. =Natural Selection.=—Survival of the Fittest in the struggle for existence. For a full account of the process see _Darwin and after Darwin_, p. 251 _et seq._ =Neo-Darwinians.=—Those who believe that Natural Selection has been the only modifying influence in the evolution of species, and that the material for its action has been only plasmogenetic characters (q. v.). =Neo-Lamarckians.=—Those who hold that organic evolution has been effected solely by means of the occurrence and preservation (inheritance) of somatogenetic characters (q. v.). =Nuclear Thread or Loops.=—_See_ Chromatin threads. =Nucleo-plasm.=—_See_ pp. 30 and 32. =Nucleus.=—_See_ Cell nucleus. =Nutritive congenital characters.=—_See_ p. 64. =Ontogenetic grades.=—_See_ p. 35. =Ontogeny.=—The life history of the individual, as distinguished from the ancestral history of the race (Phylogeny). =Ova.=—Eggs—the product of the female reproductive gland (ovary or ovarium). =Ovule.=—The seed in its earliest condition. =Pangenesis= (=Darwin=).—The theory of Heredity by gemmules (q. v.). =Panmixia= (=Weismann=).—The condition of free intercrossing, i.e. where Natural Selection (q. v.) cannot act. =Parthenogenesis.=—A degenerate form of sexual reproduction, in which the egg develops without having been fertilized by the male element. =Phylogeny.=—The ancestral history of the race, as distinguished from the life history of the individual (Ontogeny).