Page:Popular Science Monthly Volume 33.djvu/809

Rh tubes, having solid walls, and the end truncated with a membranous surface, riddled with holes. By these microscopic openings escapes the liquid which, hardened in contact with the air, becomes the thread out of which the web and the cocoon are made. Although it is cited as the type of fineness, this thread is formed of several fibers, which adhere together on issuing from the spinneret. It is unrivaled for evenness, delicacy, and power of resistance.

The internal organization of the spider is even more admirable than the external parts. It would be hardly possible even to point out in this paper the most essential features of it. It would be going into long details to describe a muscular apparatus having a power of which the animal kingdom affords few examples, assuring wonderful precision and agility in movement; a nervous system whose enormous development accounts for faculties of a superior order; and a stomach of construction peculiarly adapted to a diet composed exclusively of fluids. It is written that spiders breathe by lungs. They have an aerial respiration, but it is by organs very different in structure from the lungs of man. They consist of minute pockets containing flattened sacks packed like the leaves of a book, through the walls of which the blood infiltrates, and the interior of which is penetrated by the air. Thus observed under water, the little sacks appear like so many sheets of silver communicating with the outside by slits at the bottom of the belly. Spiders have also a heart and a circulation of blood of the most complex character. The heart, which is on the dorsal face, is of an ideal anatomical structure, and long evaded the attempts of investigators to discover the vessels that carry the blood to the periphery of the body. The main vessels were finally traced out by means of colored injections in the European species, and the smaller ones afterward in the larger South American species. The study was a most charming one to