Appendix 861 



Trial-and-error, theory that living organisms find their way to favorable environ- 

 ments by continually avoiding less favorable ones. 



Trichinella (tri ki -nel' la) (Gr. thrix, hair), small roundworm, sometimes parasitic 

 in pork, causing trichinosis in man. 



Trichocyst (trik'osist) (Gr. thrix, thread; kystos, bag), organelle producing hair- 

 like fibers for offensive and defensiv^e purposes in such animals as Para- 

 mecium. 



Tricuspid (tri -kus' pid) (L. tres, three; cuspis, point), three-pointed. 



Trihybrid (tri -hy' brid) (L. tres, three; hyhridos, mongrel), offspring of parents 

 who differ with regard to three different traits. 



Trilobite (tri' lo bite) (L. tres, three; lobos, lobes), type of extinct crustacean 

 with a trilobed body. 



Triploblastic (trip lo -bias' tik) (Gr. triplax, triple; blastos, bud), three primary 

 germ layers (ectoderm, mesoderm, entoderm) from which all organs and 

 tissues arise. 



Trochanter (tro -kan' ter) (Gr. trochanter, run), second segment of an insect's 

 leg. 



Trophectoderm (Trophoderm) (trof -ek' to durm) (Gr. trophe, food; ecto, exter- 

 nal, derma, skin or layer), outer layer of cells of the embryonic morula 

 which later supplies nourishm.ent (contrast with Inner cell mass). 



Trophozoite (trof o -zo' ite) (Gr. trephein, nourish; zoon, animal), sporozoan dur- 

 ing its growth stage. 



Tropism (tro'pizm) (Gr. trope, turn), automatic response of living organism to 

 a stimulus. 



Trypsin (trip' sin) (Gr. truein, rub down; pepsis, digest), protein-splitting enzyme 

 of the pancreas. 



Tube foot (L. tuba, pipe), tubular organ of certain echinoderms (as starfish) for 

 locomotion, etc. 



Turgor (tur' gor) (L. turgere, to swell), pressure within a cell because of absorp- 

 tion of water. 

 Twinning, production of two individuals at the same time. 

 Tympanum (tim' pan um) (L. tympanum, drum), eardrum. 



Typhlosole (tif'losole) (Gr. typhlos, bhnd; solen, channel), median, dorsal fur- 

 row of earthworm intestine to increase absorption. 



U 



Ulna (ul' na) (L. ulna, elbow), bone which together with the radius forms the 

 forearm. 



Umbilical cord (um-bil'ikl) (L. umbilicus, navel), cord composed of blood ves- 

 sels and connective tissues to connect fetus with the mother. 



Umbilicus (im -bil' i kus) (L. umbilicus, navel), scar on the abdomen where the 

 umbilical cord was attached. 



Uniformitarianism (uni form i -ta' ri an izm) (L. unus, one; forma, form), doc- 

 trine that past geologic processes were similar to those of the present (con- 

 trast with Catastrophism) . 



Unit character, trait which is inherited independently and more or less as a unit. 



