The Origin of Species by Means of Natural Selection Part 30

TRACHEA.--The windpipe or pa.s.sage for the admission of air to the lungs.

TRIDACTYLE.--Three-fingered, or composed of three movable parts attached to a common base.

TRILOBITES.--A peculiar group of extinct crustaceans, somewhat resembling the woodlice 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.

UMBELLIFERAE.--An order of plants in which the flowers, which contain five stamens and a pistil with two styles, are supported upon footstalks 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 VERTEBRAE, which const.i.tutes 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 const.i.tute 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 the parent of the product of the latter, which is often very different from that of the egg. The individuality of the species is represented by the whole of the form produced between two s.e.xual reproductions; and these forms, which are apparently individual animals, have been called ZOOIDE.