GLOSSARY. 573 



HIRUDINEA (Lat. hirudo, a horse-leech). The order of A nnelida comprising 



the Leeches. 

 HISTOLOGY (Gr. histos, a web ; logos, a discourse). The study of the tissues, 



more especially of the minuter elements of the body. 

 HOLOCEPHALI (Gr. holes, whole ; kephale, head). A sub-order of the Elasmo- 



branchii comprising 1 the Chimcerce. 

 HOLOSTOMATA (Gr. holos, whole ; stoma, mouth). A division of Gasteropodous 



Molluscs, in which the aperture of the shell is rounded, or " entire." 

 HOLOTHUROIDEA (Gr. holothourion ; and eidos, form). An order of Echinoder- 



mata comprising the Trepangs. 

 HOMOCERCAL (Gr. homos, same ; kerkos, tail). Applied to the tail of Fishes 



when it is symmetrical, or composed of two equal lobes. 

 HOMOLOGOUS (Gr. homos ; and logos, a discourse). Applied to parts which 



are constructed upon the same fundamental plan. 

 HUMERUS. The bone of the upper arm (Irachium) in the Vertebrates. 

 HYALINE (Gr. hualos, crystal). Crystalline or glassy. 

 HYBODONTS (Gr. hubos, curved ; odous, tooth). A group of Fishes of which 



Hylodus is the type-genus. 

 HYDROIDA (Gr. hudra; and eidos, form). The sub-class of the Hydrozoa, 



which comprises the animals most nearly allied to the Hydra. 

 HYDROTHECA (Gr. hudra; and tJieke, a case). The little cbitinous cups in 



which the polypites of the Serlularida and Campanularida are protected. 

 HYDROZOA (Gr. hudra; and zob'n, animal). The class of the Coelenterata 



which comprises animals constructed after the type of the Hydra. 

 HYMENOPTERA (Gr. humen, a membrane ; pteron, a wing). An order of In- 

 sects (comprising Bees, Auts, &c.) characterised by the possession of four 



membranous wings. 



HYOID (Gr. U ; eidos, form). The bone which supports the tongue in Ver- 

 tebrates, and derives its name from its resemblance in man to the Greek 



letter U. 

 HYPOSTOME (Gr. hupo, under ; stoma, mouth). The upper lip, or "labrum," 



of certain Crustacea (e.g., Trilobites). 

 HYRACOIDEA (Gr. kurax, a shrew ; eidos, form). An order of the Mammalia 



constituted for the reception of the single genus Hyrax. 



ICHTHYODORULITE (Gr. ichthus, fish ; dorus, spear ; lithos, stone). The fossil 

 fin-spines of Fishes. 



ICHTHYOMORPHA (Gr. ichthus ; morphe, shape). An order of Amphibians, 

 often called Urodela, comprising the fish-like Newts, &c. 



ICHTHYOPHTHIRA (Gr. ichthus ; phtheir, a louse). An order of Crustacea com- 

 prising animals which are parasitic upon Fishes. 



ICHTHYOPSIDA (Gr. ichthus ; opsis, appearance). The primary division of 

 Vertebrata, comprising the Fishes and Amphibia. Often spoken of as the 

 Branchiate Vertebrata. 



ICHTHYOPTEUYGIA (Gr. ichthus ; plerux, wing). An extinct order of Reptiles. 



ICHTHYOSAURIA (Gr. ichthus ; saura, lizard). Synonymous with Ichtliyopterygia. 



ILIUM. The haunch-bone, one of the bones of the pelvic arch in the higher 

 Vertebrates. 



IMAGO (Lat. an image or apparition). The perfect insect, after it has under- 

 gone its metamorphoses. 



IMBRICATED. Applied to scales or plates which overlap one another like tiles. 



INCISOR (Lat. incido, I cut). The cutting teeth fixed in the intermaxillary 

 bones of the Mammalia, and the corresponding teeth in the lower jaw. 



INEQUILATERAL. Having the two sides unequal, as in the case of the shells 

 of the ordinary bivalves (Lamellibranchiata) . When applied to the shells 

 of the Foramiuifera, it implies that the convolutions of the shell do not lie 

 in the same plane, but are obliquely wound round an axis. 



INEQUIVALVE. Composed of two unequal pieces or valves. 



INFUNDIBULUM (Lat. for funnel). The tube formed by the coalescence or 

 apposition of the epipodia in the Cephalopoda. Commonly termed the 

 "funnel," or "siphon." 



