Fig. 33.—Mouth parts of Chilognatha. (From C. L. Koch, System der Myriapoden.) f and g, The mandibles. The parts marked a, b, c, d, e are firmly united and constitute the broad plate No. 3. They have received the following names—a, b, Internal stipes; c, external stipes; d, malellae; e, hypostoma.
After the mouth parts we come to the legs. We first notice the fact that the bases of the legs in each pair are closely approached to one another. They are so set into the body that the basal joints, or, as they are called, the coxal joints, nearly touch. This is the case in almost all Chilognatha, except in the Polyxenidae, and it is a fact connected with some important features in the internal anatomy. Then we have the peculiarity in the Chilognatha which has formed the basis of most classifications which have placed these animals in a group by themselves. This is the possession in most segments of two pairs of legs. This characteristic has caused the group to be called by some naturalists Diplopoda. As a general rule, the first four segments have only three pairs of legs between them, one of them being without a pair of legs. This legless or apodal segment is usually the third. From the fifth segment to the end of the body all the segments have two pairs of legs each. The legs are shorter than those of the Chilopods, and are all nearly equal in size. This is not the case in the other Orders. The legs are commonly wanting in the seventh segment of the male, and are replaced by a copulatory organ. This peculiarity is related to the different position of the generative openings in the Chilognatha. Another anatomical feature peculiar to the Chilognatha is the possession of the stink glands—the glandulae odoriferae before mentioned. This, however, is a character which does not hold for all the Chilognatha, since the Polyxenidae have none of these glands. All the other families, however, possess them, and they are present in none of the other Orders.
As regards the internal anatomy of the Chilognatha, the digestive canal differs mainly in the glands which supply it with secretions. It receives the saliva from two long tubular salivary glands, which open at the base of the four-lobed plate which has been mentioned as the third of the mouth appendages. The secretion of these glands is used, as has already been said, in the process of preparing the nest for the eggs. We cannot fail to be reminded of a similar function of salivary glands in those swallows, which prepare the nests of which bird's-nest soup is made with the secretion of the salivary glands. Another feature in the form of the digestive tube is that in many cases, if not in all, it is marked with constrictions which correspond with the segments of the body.
The heart in the Chilognatha is not such a highly developed organ as in the other Orders. The muscles which have already been mentioned as the alary muscles (or wing-shaped muscles) are not so highly developed, and consist for the most part of a few muscular fibres. The muscular walls of the heart, which consist of three layers, have the muscles less strongly developed, and are in general adapted for a less energetic circulation.
The tracheae, which open into the stigmata, as has already been said, branch into tufts of fine tubes, but the ramifications of these tufts never join (or anastomose, as it is called), and consequently we never get, as in the other Orders, long tracheal trunks running along the body.
The nervous system, in addition to the existence of the visceral nerve system described by Newport, shows a marked peculiarity in the form of the ventral ganglionic chain. As has already been said, the nerve system consists of a brain or mass of ganglia fused together and connected with the ventral nervous cord by a collar of nervous substance surrounding the oesophagus, and generally known as the circumoesophageal collar. The ventral nerve cord is a stout cord of nervous substance passing along the whole length of the animal, and situated below (or ventral to) the digestive tube and the generative system. This cord is enlarged at certain points, and these enlargements or swellings are called ganglia, while from the ganglia pass off nerves which supply the different organs of the body. In the Chilognatha the cord has a compressed appearance as if the ganglia were pressed into one another in such a way that it is very hard to distinguish any ganglia at all. If we use the microscope and examine sections cut transversely through the cord, we see that it is not a simple cord. Even if we examine the nerve cord with a simple lens, we see that a furrow runs longitudinally down it, and the use of the compound microscope shows us that this furrow represents a division into two cords in such a way that the single stout cord as it appeared to the naked eye is in reality two cords running side by side, and so compressed together that the substance is partly fused together. The ganglia too are double, being swellings of the two cords and not a single enlargement on a single cord. As we shall see in the other Orders, this arrangement constitutes a characteristic distinction.
The generative organs consist of a long tubular ovary or testis lying along almost the whole length of the body and placed between the digestive organ and the nervous system. Near its exit from the body the long tube divides into two short tubes, the oviducts in the female or the vasa deferentia in the male. These ducts open in the third segment of the body, unlike those of Myriapods belonging to other Orders. The accessory glands present in most other Myriapods are not present in the Chilognatha.
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 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.
