Page:The spiders of the northern states-1901.djvu/6

 INTRODUCTION.

The particular differences between the lower animals are generally but little observed. It is, therefore, well, as introductory to the study of a certain set of animals, to observe the place they occupy in the systematic arrangement of Natural History. This draws the attention of the student to their peculiarities, and, at the same time acquaints him with all related and resembling families.

The animal world has been divided into two parts,—animals without a backbone (Invertebrate) and those with a backbone (Vertebrate). The first part reaches from the lowest infusoria, hardly distinguishable from plant life, to the most perfect insect. It is divided into several classes, of which the most highly developed is that of jointed animals (Arthropoda). The bodies of these are encased ina series of rings, variously grouped and jointed together, which serve instead of bones to keep the body in form. This class consists of three orders,—the Myriopoda or Centipedes, with a head and a row of equal rings, many legs and no wings; the /vsecta or true insects, with the rings grouped in three parts, head, breast part and body part. All have six legs, a pair of feelers, (antennw) in their faces between the eyes, and most of them have wings.

The order of the Arachnida, spider-like animals, is placed between the myriopods and insects. They have eight legs, no feelers between their eyes, but often a pair of leg-like feelers from their mouth parts (palpi) and no wings. The short-legged mites (Acarina) and long-legged harvestmen (Phalangide) have their head, chest and body part united to an oval-shaped body. The scorpions have a pair of