424 THE INVERTEBRATA 



segment. Also a postlabial segment contributes to it forming the 

 basilar plate. The tergite of this segment, however, forms what is ap- 

 parently the first segment after the head. This is known as the collum\ 

 there are no stigmata and no separate appendages, though the first pair 

 of legs appears to belong to it they are those of the second segment. 

 The next three have a single pair of ambulatory legs apiece, a pair of 

 ganglia and a pair of stigmata, and in the embryo a pair of coelomic 

 sacs. These four segments maybe said to constitute the thorax, though, 

 as related above, the first takes part in the formation of a head 

 structure. The genital openings are situated in the basal joint of the 

 second pair of legs, which appear to arise from the second segment, 

 but really belong to the third. 



Behind this is the abdomen consisting of an indefinite number of 

 double segments (up to a hundred in lulus). The exoskeleton of a 

 body segment consists of a tergum and two sterna. In the double 

 segment of lulus (Fig. 299 B) the sclerites of two segments are fused 

 together to make a continuous ring. The sterna carry two pairs of 

 stigmata and legs. In the embryo there are two pairs of coelomic 

 sacs ; there are two ostia in the heart and two pairs of ganglia. In 

 lulus the sterna are much shorter than the terga and also much 

 narrower so that the legs come oif very close together ; also the terga 

 are narrower in front so that they can be telescoped into the terga in 

 front. The diagram here given (Fig. 299 D) shows that this relation 

 occurs when the diplopod body is straightened out ; when the animal 

 rolls up the adjacent rings are completely disengaged. 



The stigmata are elongated slits, which can be closed by a valve, 

 and they communicate with a tracheal pocket from which spring two 

 thick bunches of unbranched tracheae. These are of two sorts; one 

 long and slender, the other shorter and thicker with a spiral lining. 

 In other millipedes {Glomeris) the tracheae may become much longer 

 and branch but they never anastomose. 



The circulatory system is in a stage of development higher than 

 that of the insects. The alimentary canal bears a pair of long 

 Malpighian tubules arising from the hind gut. 



The legs consist of the same elements as in the insect, but the 

 tarsus is divided into three joints, the last of which carries a claw. In 

 the male the first leg is modified for copulation and in the 7th segment 

 there is an auxiliary copulatory apparatus, consisting of processes 

 used for transferring sperm into the vagina of the female. These 

 processes may occur together with legs and so are not homologous 

 with them. There are no similar organs in the female. The generative 

 glands are unpaired with ducts opening on the 3rd body segment. The 

 eggs are yolked and are laid after copulation in a nest made of hard 

 earth. The mother keeps watch over them before hatching. 



