GLOSSARY 499 



Total Solids All of the solids in the water, including dissolved, 

 suspended, and settleable components. 



Toxicity A relative measure of the ability of a chemical to be toxic. 

 Usually refers to the ability of a substance to kill or cause an adverse 

 effect. High toxicity means that small amounts are capable of causing 

 death or ill health. 



Toxicology The study of the interactions between organisms and a toxi- 

 cant. 



Toxin A particular class of poisons, usually albuminous proteins of high 

 molecular weight produced by animals or plants, to which the body 

 may respond by the production of antitoxins. 



Transmission The transfer of a disease agent from one individual to 

 another. 



Transmission, Horizontal Any transfer of a disease agent between indi- 

 viduals except for the special case of parent-to-progeny transfer via 

 reproductive processes. 



Transmission, Vertical The parent- to- progeny transfer of disease 

 agents via eggs or sperm. 



Trauma An injury caused by a mechanical or physical agent. 



Trematoda The flukes. Subclass Monogenea: ectoparasitic in general, 

 one host; subclass Digenea: endoparasitic in general, two hosts or 

 more. 



Tumor An abnormal mass of tissue, the growth of which exceeds and is 

 uncoordinated with that of the tissues and persists in the same exces- 

 sive manner after the disappearance of the stimuli that evoked the 

 change. 



Turbidity Presence of suspended or colloidal matter or planktonic 

 organisms that reduces light penetration of water. 



Turbulence Agitation of liquids by currents, jetting actions, winds, or 

 stirring forces. 



Ubiquitous Existing everywhere at the same time. 



UDN See Ulcerative Dermal Necrosis. 



Ulcer A break in the skin or mucous membrane with loss of surface tis- 

 sue; disintegration and necrosis of epithelial tissue. 



Ulcer Disease An infectious disease of eastern brook trout caused by 

 the bacterium Hemophilus piscium. 



Ulcerative Dermal Necrosis (UDN) A disease of unknown etiology 

 occurring in older fishes, usually during spawning, and primarily 

 involving salmonids. 



United States Pharmacopeia (USP) An authoritative treatise on 

 drugs, products used in medicine, formulas for mixtures, and chemical 

 tests used for identity and purity of the above. 



