470 GENERAL ZOOLOGY 



Hypothesis. A tentative supposition provisionally adopted for explaining 

 certain facts and serving as a guide for further investigation. A 

 stage in the development of a theory. 



Hypotonic. Having a lower osmotic pressure than normal. 



Immunity. The resistance of the body to infection by pathogenic organ- 

 isms, natural or acquired, through the production of antitoxins. 



Infection. Implantation of disease, pathogenic organisms, or parasites 

 from without. 



Insemination. The addition of sperm to eggs. The introduction of sperm 

 into a female. 



Integrative action. Said of the function of the nervous system as a 

 coordinating mechanism for unifying bodily activities. 



Intercellular. Between cells. 



Internal respiration. The interchange of oxygen and carbon dioxide 

 between tissues and the circulatory fluid. True respiration. 



Internal secretion. Usually the product of a ductless gland that is 

 absorbed by the blood. Hormone. Endocrine. 



Intracellular. Within a cell. 



Intussusception. Growth by the intercalation of substances throughout 

 the cells of an organism, as contrasted with growth by accretion- 

 deposits of particles on the outside — such as occurs in crystals. 



Invagination. The ingrowth of a pocket of cells from a surface. 



Invertebrate. An animal without a backbone. 



Ion. An atom or group of atoms bearing an electrical charge. Hydrogen- 

 ion concentration refers to the number of free hydrogen ions in a solu- 

 tion, which determines whether the solution is "acid" or "basic" in 

 reaction. 



Irritability. The power of protoplasm to respond to stimuli. 



Isotonic. Having the same osmotic pressure. 



Karyokinesis. The indirect method of cell division; mitosis. 



Kinetic energy. Energy possessed by a body by virtue of motion. 

 Manifested by heat production in chemical reactions. 



Lacteal. A lymph vessel of the intestine. 



Lacuna. An intercellular space. 



Lamellated. Arranged in layers. 



Lamellibranchiate. Belonging to a group of molluscs having platelike 

 gills, such as the clam, oyster, mussel, etc. 



Larva. A usually active stage in the development of an animal marked 

 by the presence of larval organs and by the absence of adult ones. An 

 inmature but free-living stage in development. 



Lateral. Pertaining to the side. 



Law. A statement of an order or relation of scientific facts which, so far 

 as known, is invariable. 



Lesion. A wound or local degeneration. 



Linear. Arranged in a line or row. 



Lumbar. Pertaining to the region of the back posterior to the thorax. 



Lumen. Cavity or passageway of a tubular structure. 



Lymph. In vertebrates a circulatory fluid similar to blood but lacking red 

 corpuscles. Same as blood in many invertebrates. 



