The general arrangement of the organs of the Chilognatha may be seen from Fig. 34, which represents a transverse section through the body of _Polyxenus_ (Fig. 18). A comparison of {56}these two figures (Figs. 34 and 18) will show the position of the organs mentioned in this account. The heart is shown with the suspensory and alary muscles attached. [Illustration: FIG. 34.—Transverse section through _Polyxenus lagurus_: _g.n.c_, _f.n.c_, ganglionic and fibrous parts of nerve cord; _Rec.sen_, receptaculum seminis; _ori.dct_, oviduct; _Spmzoa_, spermatoza. (From Heathcote, Anatomy of _Polyxenus lagurus_.)] ORDER II. CHILOPODA. The shape of the body differs from that of the Order which has been just described (Chilognatha), inasmuch as it is not cylindrical but flattened, the back, however, being more arched than the ventral surface. In this respect, however, it cannot be said to differ from the other Orders which we have yet to describe. The segments are not formed by a single ring of the exoskeleton, which in this Order is formed of chitin, and is tough and flexible rather than hard and strong; but of two or three plates which form a covering to the segment. The back is covered by a large plate known as the _tergum_, the sides by two plates known as _pleura_, and the ventral part by a plate called the _sternum_. The pleura and sternum are, however, in most cases fused together or indistinguishable. In this, as in most of the anatomical peculiarities, there is a much greater difference between the two Orders Chilopoda and Chilognatha than between {57}the Chilopoda and the other three Orders which have still to be described. The Chilopoda have only one pair of appendages to each segment of the body instead of two pairs like the Chilognatha. The antennae of the Chilopoda are as a rule very long, and are always longer than in the Chilognatha which we have just described. They differ from those of the Schizotarsia (the third Order, which will be the next to be described) in having the basal joints nearer together; in other words, they are differently placed on the head. They differ from those of the Pauropoda (the fifth Order) in being straight and not branched. As a rule the antennae of the Chilopoda taper towards the extremity. [Illustration: FIG. 35.—Mouth parts of _Lithobius_ (Latzel). A, Head of _Lithobius_ seen from the under surface after removal of poison claws; _a_, second maxilla; _b_, _c_, the two shafts of the first maxilla. B, One of the mandibles. C, The two poison claws.] The mouth parts are more numerous than in the Order we have just described (the Chilognatha). They consist of— 1. An upper lip. This is a transverse plate as just described in the case of the Chilognatha, but it is not always fused with the rest of the head. It is also usually composed of three pieces, two lateral and a middle piece.