GLOSSARY 383 



Hexiology Qieks' i oV oji). The science of the relation of animals and plants to their 



environment; sometimes erroneously spelled hexicology. 

 Hipparion (hip pa' ri on). An extinct horse-like animal of Miocene and Pliocene time 



in North America and Europe. 

 Hippocampus {hip' po kam' pus). A genus of fishes of bizarre form resembling 



in part a horse's head. 

 Hirudinea (hi' ru din' e a). A class of Annelida comprising the leeches. For defini- 

 tion see Chapter XII. 

 Histology (his toV o ji). The science which deals with the structure of tissues. 

 Holothurioidea (ho' lo thu' rioi' dea). A class of Echinodermata, comprising the sea 



cucumbers. For definition see Chapter XII. 

 Homo {ho' mo). The genus of animals comprising man. 

 Homolecithal {ho' mo les' i thai). Having the yolk uniformly distributed throughout; 



said of eggs. 

 Homologous {ho moV o gus). Originating in the same way in evolution and hence 



in the embryo ; said of organs or structures. 

 Homology {ho moV oji). Similarity of origin in evolution and hence in the embryo; 



applied to organs that arise in the same way. 

 Homozygote {ho' mo zi' gate). An organism whose two parents contributed to it 



similar genes for some inherited character, and whose germ cells are therefore all 



alike with respect to that character. 

 Homozygous {ho' mo zi' gus). Of the nature of a homozygote. 



Hooke, Robert {hook). English natural philospher and mathematician, 1635-1703. 

 Hooker, Sir Joseph Dalton {hook' er). English botanist, 1817- 1911. 

 Hormone {hor' mo ne or hor' mone). A secreted substance which stimulates activity 



in an organ. 

 Humerus {hu' mer us). The single bone of the upper arm in Amphibia and the higher 



vertebrates. 

 Huxley, Thomas {huks' li). EngUsh biologist and lecturer, 1825-1895. 

 Hyaliodes {hi' a li o' deez). A genus of beetles. 

 Hybrid {hi' brid). The offspring of two parents unlike one another in some heritable 



character. 

 Hybridization {hi' brid i za' shun). The process of crossing animals having unlike 



heritable characters, thereby producing animals possessing genes for the traits of 



both parents. 

 Hydra {hi' dra). A small tubular freshwater animal with tentacles and stinging 



organs, belonging to the phylum Ccelenterata. Two species are common, H. oli- 



gactis {ol' i gak' tis), and H. viridissima {veer' i dis' si ma). 

 Hydractinia {hi' drak tin' i a). A genus of marine colonial and polymorphic Hydro- 



zoa, living attached to the snail shells occupied by hermit crabs. 

 Hydranth {hi' dranth). A Hydra-like, tentacle-bearing member of a hydroid colony. 

 Hydrocorallinae {hi' dro kor' al W nee). An order of Hydrozoa comprising certain 



corals. 

 Hydroid {hi' droid). A colonial coelenterate, the individuals of which resemble Hydra 



in certain respects. 

 Hydrolysis {hi droV i sis) . A double chemical decomposition in which one of the 



substances consumed is water. 

 Hydrolyze {hi' dro lize). To undergo, or to subject to, hydrolysis. 



