54 MYRIAPODA CHAP. 



liave 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 Chilo- 

 gnatha. Another anatomical feature peculiar to the Chilognatha 

 is the possession of the stink glands the glandulae odoriferae 

 before mentioned. This, however, is a character wdiich does not 

 hold for all the Chilognatha, since the Polyxenidae have none of 

 these glands. All the other families, however, possess them, 

 except the Chordeumidae. 



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 con- 

 sequently we never get, as in the other Orders, long tracheal 

 trunks running along the body. 



