[Illustration: Fig. 72.--WEB OF THE GEOMETRICAL SPIDER.] The chief instrument by which the spider performs these wonders is the spinning apparatus. The matter from which the threads are spun is the liquid contained in cells; the ducts from these cells open upon little projecting teats, and the atmosphere has so immediate an effect upon this liquid, that upon exposure to it the secretion becomes a tough and strong thread. Twenty-four of these fine strands form together a thread of the thickness of that of the silk-worm. We are assured that there are three different sorts of material thus produced, which are indeed required for the various purposes to which they are applied--as, for example, to mix up with the earth to form the cells; to line these cells as with fine cotton; to make light and floating threads by which they may be conveyed through the air, as well as those meshes which are so geometrically and correctly formed to entrap their prey.--_Note by Lord Brougham to Paley's Natural Theology._ [Verse: "For every beast of the forest is mine, and the cattle upon a thousand hills."--PSALM L.] 1083. _Why have many insects a great number of eyes?_ Because the orb of the eye is fixed; there is therefore placed over the eye a multiple-lens, which conducts light to the eye from every direction; so that _the insect can see with a fixed eye as readily as it could have done with a movable one_. As many as fourteen hundred eyes, or inlets of light, have been counted in the head of a drone-bee. The spider has _eight eyes_, mounted upon different parts of the head; two in front, two in the top of the head, and two on each side. 1084. _Why have birds of prey no gizzards?_ Because their food _does not require to be ground_ prior to digestion, as does the food of grain-eating birds. 1085. _Why have earth worms no feet?_ Because the undulatory motion of their muscles serves them for fill the purposes of progression _needed by their mode of life_. 1086. _Why have mussels strong tendinous threads proceeding from their shells?_ Because as they live in places that are beaten by the surf of the sea, they _moor their shells_ by those threads to rocks and timbers. 1087. _Why have cockles stiff muscular tongues?_ Because, having no threads to moor themselves, as the mussels have, they _dig out with their tongues a shelter for themselves in the sand_. 1088. _Why do oxen, sheep, deer, &c., ruminate?_ Because they have no front teeth in the upper jaw, the place of which is occupied by a hardened gum. The first process, therefore, consists simply of _cropping_ their food, which is passed into the paunch, to _be brought up again and ground by the back teeth_ when the cropping process is over.