Arthropods almost entirely on the ventral side. In the head is situated the cerebral ganglion, a large nervous mass resting on the gullet, and giving off nerves to the eyes and feelers. Besides this, there is a ventral nervecord on the ventral side of the animal, running below the gut, and made up of several pairs of ganglia, united with one another by means of nerve fibres. The ganglia of the ventral cord send their nerves to the jaws, limbs, muscles, viscera, etc. The cerebral ganglion is connected with the first ventral ganglion by means of a cord on each side of the gullet (Fig. 56), so that a ring-like structure is formed. The breathing organs are gills in some Arthropods (the Crustacea, _e.g._ crayfish, crabs); insects and centipedes breathe by air-tubes or tracheæ, while the respiratory organs of spider-like animals, when these do not breathe entirely by means of the skin, are more or less strongly modified tracheæ. The structure of the tracheal system is generally as follows: on each side of the body there is a row of breathing-holes, or stigmata, through which the air can enter the tracheæ; these are very much branched, so that they finally become very fine tubes investing the various organs, which in this way obtain the requisite amount of oxygen. The stigmata on one side of a caterpillar are clearly shown in Fig. 60, and those of a hornet larva in Fig. 61. Four classes belong to this sub-kingdom: Insecta (Insects), Myriapoda (centipedes, etc.), Arachnoidea (spiders, scorpions, etc.), Crustacea (crayfish, crabs, lobsters, etc.). CLASS I.: =INSECTA= (INSECTS). [Illustration: FIG. 57.—A Grasshopper, with the different regions of the body separated from one another. A, head; B, thorax: _I._, prothorax; _II._, mesothorax; _III._, metathorax; C, abdomen; _a_, antennæ; _b_, eyes; _c_, _d_, _e_, legs; _f_, _g_, wings; _p_, shank; _q_, foot. ] Breathe by tracheæ (cp. p. 84). The segments fuse into three body regions (Fig. 57). These are—(1) the _Head_, bearing the eyes, feelers (antennæ), and jaws; (2) the _Thorax_, composed of three fused segments, of which the first (prothorax) bears a pair of legs, while the second (mesothorax) and third (metathorax) not only bear a pair of legs each, but also sometimes a pair of wings in addition; (3) the _Abdomen_, which possesses no limbs, and of which the number of segments is not always the same. These three regions of the body perform different functions; the head is concerned with sensation and the taking up of food, the thorax with movement, while the abdomen contains the organs of digestion and reproduction. I will now deal somewhat more fully with the different body regions of an insect.