One of the most important characteristics which distinguish Myriapods from other Arthropoda is the fact that they possess on the posterior segments of the body true legs which are jointed and take part in locomotion. The head is in all cases quite distinct from the body, and may be regarded as a number of segments fused together into one mass. Their heads are always provided with a single pair of _antennae_ and mouth appendages, consisting of an upper lip, a pair of _mandibles_ or jaws, and one to two pairs of _maxillae_. The mandibles resemble those of Insects, and are strongly toothed. In the Chilognatha a pair of maxillae are fused so as to form a single oval appendage. In the Chilopoda they each consist of a single blade bearing a {48}short palp or feeler. The mouth parts may have the forms known as chewing, biting, or suctorial (_Polyzonium_) mouth appendages. With the exception of the terminal segment, and in many cases the first or the seventh, each segment bears one or two pairs of limbs. These may be very long, as in _Scutigera_, or very short, as in _Polyxenus_. They may be attached close to one another near the ventral middle line of the body, or may have their bases far apart from each other, as in the Chilopoda. The exoskeleton or external armour is composed of chitin (Chilopoda) or of chitin with calcareous salts deposited in it (Chilognatha). Their internal structure has a great likeness to that of Insects. The general position of the internal organs may be seen from Fig. 28, which shows a _Lithobius_ dissected so as to exhibit the digestive and nervous systems. _The digestive canal_, which is a straight tube, extends throughout the whole length of the body, and terminates in the last segment of the body. It may be divided into the following parts:— 1. A narrow oesophagus, beginning with the mouth or buccal cavity, and receiving the contents of two or more salivary glands (_d_). 2. A wide mesenteron or mid-gut (_n_) extending throughout almost the whole length of the body. 3. A rectum which at its junction with the mid-gut receives the contents of two or four _Malpighian tubes_ (_g_, _h_) which function as kidneys. Their function was for a long time unknown, but the discovery of crystals of uric acid in them placed the matter beyond doubt. _The heart_ has the form of a long pulsating dorsal vessel which extends through the whole length of the animal. It is divided into a number of chambers, which are attached to the dorsal wall of the body, and are furnished with muscles of a wing-like shape, which are known as the alary muscles, and which govern its pulsations. The chambers are furnished with valves and arteries for the exit of the blood, and slits known as _ostia_ for the return of the blood to the heart. The blood enters the chambers of the heart from the body cavity through the {49}ostia, and passes out through the arteries to circulate through the organs of the body and to return by the ostia.