2. A pair of jaws or mandibles, which are not of so simple a form as those of the Chilognatha, but rather resemble those of some of the Crustacea.
3 and 4. Two pairs of appendages called maxillae resembling feet, but used to aid the act of eating instead of locomotion. They are very different in different Chilopods, but are mostly slender and weak and usually provided with feelers (or palps) growing out of the main stem.
5. The next pair of appendages are the first pair of the legs of the body, which are also metamorphosed to serve a function different from the ambulatory function of the other limbs. These are the poison claws, and the possession of these forms another distinction between the Order we are now discussing and that of the Chilognatha. At the same time the third Order, that of the Schizotarsia, has poison claws, so that this feature does not separate the Chilopoda from all the other Orders. These poison claws are large curved claws connected with poison glands, the secretion of which flows through a canal which opens near the point.
The legs are longer than those of the Chilognatha, but not so long as those in the next Order to be described (the Schizotarsia). Their number is very various, from 15 pairs in Lithobius to 173 in the Geophilidae. Latzel notes a curious point in the number of the legs in this Order, namely, the number of pairs of legs is always an uneven one. There are always one pair to each segment. The last pair of legs is always longer than the other pairs, and this is a peculiarity of the Order.
The digestive tube resembles that of the other Orders, but the salivary glands are not long and tubular but short (Fig. 28, d). It is, moreover, not marked with constrictions corresponding with the segments of the body.
The tracheal system or the system of respiration may be said to be more highly developed in this Order than in any other. The tracheal branches anastomose with one another (that is, the branches join), and in some cases form long tracheal stems running along the body almost for its whole length. The number of the tracheal openings or stigmata varies and does not correspond with the number of segments.
The nervous system differs considerably from that in the Order Chilognatha; it resembles that in the Schizotarsia, and differs again from that in the other two Orders, Symphyla and Pauropoda. The brain shows some differences from other Orders chiefly in the development of the different lobes which are connected with the sense organs, the eyes and antennae, for instance; but the most marked difference is in the ventral ganglionic cord. First, the ganglionic swellings are much more clearly marked than in the Chilognatha. Secondly, the first three ganglia differ from the others in being nearer to one another and forming a single mass when seen by the naked eye, though when examined by the aid of a microscope we can see all the different parts are there. Thirdly, the division into two cords mentioned in the Chilognatha is carried to a much greater extent. The ganglia in each segment can be seen plainly to be double, and the cords connecting the ganglia are two in number. We can plainly see that the ventral nervous system of the Chilopoda consists of two cords lying parallel to one another, and each having a ganglionic enlargement in every segment. Whether a visceral nervous system is present in the group is doubtful.
The eighth family of the Chilognatha, the Polyxenidae, show an approach to the Chilopod nervous system.
The generative system differs chiefly in the opening of the genital apparatus at the end of the body instead of in the third segment; though this difference only separates the Order from the Chilognatha and not from the other Orders. They also have two pairs of large accessory glands (as they are called) connected with the genital openings.
Order III. Schizotarsia.