466 THE ORIGIN OF SPECIES 



Tentacula or Tentacles. — ^Delicate fleshy organs of prehension or 

 touch possessed by many of the lower animals. 



Tertiary. — The latest geological epoch, immediately preceding the 

 establishment of the present order of things. 



Trachea. — ^The windpipe or passage for the admission of air to the 

 lungs. 



Tridactyle. — Three-fingered, or composed of three movable parts at- 

 tached to a common base. 



Trilobites. — A peculiar group of extinct Crustaceans, somewhat re- 

 sembling the Wood-lice in external form, and, like some of them, 

 capable of rolling themselves up into a ball. Their remains are 

 found only in the Palaeozoic rocks, and most abundantly in those of 

 Silurian age. 



Trimorphic. — Presenting three distinct forms. 



UMBELLiFERiE. — ^Ah Order of plants in which the flowers, which contain 

 five stamens and a pistil with two styles, are supported upon foot- 

 stalks which spring from the top of the flower stem and spread out 

 like the wires of an umbrella, so as to bring all the flowers in the 

 same head (umbel) nearly to the same level. (Examples, Parsley 

 and Carrot.) 



Ungulata. — ^Hoofed quadrupeds. 



Unicellular. — Consisting of a single cell. 



Vascular. — Containing blood-vessels. 



Vermiform. — ^Like a worm. 



Vertebrata; or Vertebrate Animals. — ^The highest division of the 

 animal kingdom, so called from the presence in most cases of a 

 backbone composed of numerous joints or vertebrcB, which con- 

 stitutes the centre of the skeleton and at the same time supports 

 and protects the central parts of the nervous system. 



Whorls. — ^The circles or spiral lines in which the parts of plants are 



arranged upon the axis of growth. 

 Workers. — See Neuters. 



ZoEA-STAGE. — ^The earliest stage in the development of many of the 

 higher Crustacea, so called from the name of Zoea applied to these 

 young animals when they were supposed to constitute a peculiar 

 genus. 



ZooiDS. — In many of the lower animals (such as the Corals Medusae, 

 etc.) reproduction takes place in two ways, namely, by means of 

 eggs and by a process of budding with or without separation from 



