The nervous system has a large ganglion, J, in the thorax, from which branches, _i_, pass to the limbs and abdomen. At the front end two branches extend upward, each side of the œsophagus, to two smaller ganglia, _k_, from which pass nerves to the mandibles and eyes. The reproductive organs, S, lie along the under side of the abdomen, and open between the two air-sacs. The spinning-glands, _u_, lie above the spinnerets, and along the under side of the abdomen. They will be more fully described in the chapter on spinning. POISON-GLANDS. The poison-glands, _n_, are partly in the basal joints of the mandibles, and partly in the head, and discharge by a tube which opens at the point of the claw of the mandible, Fig. 15, _a_. CLASSIFICATION. There is not room in this book to explain the classification of spiders into genera and species; but a description of the following well-marked groups, which contain nine-tenths of all spiders, will give a general idea of the differences among them, and help to understand what follows. [Illustration: Fig. 6.] MYGALIDÆ This family includes the largest known spiders. The body is usually very hairy and dark-colored. Most species have only four spinnerets; and one pair of these are long, and are turned up behind the abdomen. They have four air-sacs under the front of the abdomen, instead of two, as other spiders. Their mandibles are very large, and work up and down, instead of sidewise. The eyes are collected together on the front of the head. They live only in warm countries. Specimens from South America are exhibited in every natural history museum. Fig. 6 represents _Mygale Hentzii_, a species living in Arizona and Texas. DYSDERIDÆ. A small family of spiders with only six eyes. They have also four breathing-holes in the front of the abdomen; but one pair leads to branched tubes instead of sacs. They are usually found under stones, with their legs drawn up close to their bodies, but can move very quickly when so inclined. Very few species are known, and none are common, in North America. Fig. 7 is _Dysdera interrita_ enlarged. Below are the eyes as seen from in front. DRASSIDÆ. A large family of spiders, varying greatly in shape, color, and habits. Most of them are dull colored, and live under stones, or in silk tubes on plants, and make no webs for catching insects. Their eyes are small, and arranged in two rows on the front of the head. Their feet have two claws and a bunch of flat hairs. The spinnerets are usually long enough to extend a little behind the abdomen. Fig. 8 is a _Drassus_, and the eyes as seen from in front. [Illustration: Fig. 7.] [Illustration: Fig. 8.] AGALENIDÆ. Long-legged, brown spiders, with two spinnerets longer than the others, and extending out behind the body. They make flat webs, with a funnel-shaped tube at one side, Fig. 24, in which the spider waits. Fig. 9 is _Agalena nævia_, the common grass spider. [Illustration: Fig. 9.] CINIFLONIDÆ.