GLOSSARY 385 



Hooker, Sir Joseph Dalton. English botanist, 1817-1911. 



Hormone [hor' mone). A secreted substance which stimulates activity in an 



organ. 

 Humerus. The single bone of the upper arm in Amphibia and the higher vertebrates. 

 Huxley, Thomas. English biologist and lecturer, 1825-1895. 



Hybrid. The offspring of two parents unlike one another in some heritable character. 

 Hybridization. The process of crossing animals having unlike heritable characters, 



thereby producing animals possessing genes for the traits of both parents. 

 Hydra. A small tubular fresh-water animal with tentacles and stinging organs, 



belonging to the phylum C'oelenterata. 

 Hydranth. A hydralike tentacle-bearing member of a hydroid colony. 

 Hydroid. A colonial coelenterate, the individuals of which resemble the hydra in 



certain respects. 

 Hydrorhiza {hi' dro ri' za). That part of a hydroid colony which is attached to the 



substratum. 

 Hydrotheca. The tough transparent sheath surrounding a hydranth of a hydroid; 



an expansion of the perisarc. 

 Hydroxy! ion [hi droks' il). The radical 0H~ produced in solutions of bases. 

 Hydrozoa. A class of Coelenterata, including Hydra, the hydroids, jelly fishes, and 



some corals. For definition see Chap. 19. 

 Hyla. A genus of tree frogs. 

 Hyoid. A bone or group of bones or cartilages located at the base of the tongue or 



in a corresponding situation. 

 Hypodermis. An external layer of cells beneath a secreted cuticle, as in the earth- 

 worm and in insects and Crustacea. 

 Hypohippus. An extinct horselike animal of Miocene time in North America. 

 Hypostome. A projection from the center of the circle of tentacles in a hydra or 



one of the hydroids. It is perforated by the mouth. 



Ileum {iV e um). The last and usually longest of three divisions of the small intestine. 

 Ilium {iV i um) {pL, ilia). The dorsal bone of the pelvic girdle in Amphibia and the 



higher vertebrates. 

 Incisor. One of the front cutting teeth of a mammal. 



Incubation. The warming of eggs, resulting in acceleration of their development. 

 Indeterminate. Not leading necessarily to a given end result from a given beginning: 



said of development in which cleavage cells may, if disturbed, produce some 



structure other than that which they would have produced without interference. 

 Ingestion. The taking in of food. 

 Inhalent. Breathing in; applied to one of the siphons of clams and mussels, to 



certain pores of sponges, and to other passages. 

 Innominate bone. The single bone formed by the fusion of three bones of the pelvic 



girdle in man. This name is not usually applied in the case of other vertebrates, 



though fusion of the bones of the girdle commonly occurs. 

 Insecta. A class of Arthropoda having one pair of antennae, three pairs of legs, and 



tracheae for respiration; the insects. 

 Insectivore. Technically, a mammal of the order Insectivora, including the moles, 



shrews, and hedgehogs. In a popular sense, any insect-eating animal. 

 Insertion. The place of attachment of the distal end of a muscle or its tendon. 

 Insulin (m' su lin). An endocrine secretion produced by the islands of Langerhans 



in the pancreas; its function is control of sugar metabolism. 

 Internal respiration. The transfer of oxygen from the blood to the surrounding cells; 



true respiration. 



