DIPLOPODA. 585 



two pairs of legs. An exception to this is formed usually 

 by the last three segments and in the sexually mature male by 

 the seventh segment, these being apodal or with one pair of legs 

 only. The number of segments varies from 100 or more in the 

 elongated Polyzonidae and Julidae to less than 25 in the Poly- 

 xenidae and Glomeridae. It may vary in the same species, or 

 the number may be constant, the limit of growth in this respect 

 being presumably attained at a comparatively early period of 

 life. 



In most forms the anterior part of each segment is narrower 

 than the posterior and penetrates a little way into the segment 

 in front of it in a telescopic manner. The segments carrying 

 two pairs of legs are to be regarded as having arisen by the 

 fusion of two segments (p. 589), a trace of this union being 

 often present as a transverse furrow which marks them into 

 an anterior and posterior portion. 



Each segment is covered by definite chitinous plates. There 

 is an arched plate the tergite (tergum), usually single, which 

 covers the back and extends on to the sides. There are two 

 lateral pieces, the pleura ; and one or two pairs of ventral plates, 

 the sternites (laminae pedigerae) , which carry the legs and the 

 tracheal openings. The sternites may be united across the 

 middle to form a single plate (Polydesmidae). These plates may 

 be separate and movable upon one another, or they may be 

 partly united with each other, or completely united to form a 

 continuous ring. The last segment consists of a tergite, an 

 unpaired anal plate (? sternite) and two anal valves which 

 enclose the anus. 



The legs have normally 6 or 7 joints, named as follows : coxa 

 trochanter, femur, tibia, tarsus 1, tarsus 2, tarsus 3 ; tarsus .'} 

 carries a claw. They have approximately the same size and 

 shape. The legs of a pair are usually attached close together 

 to their sternite, but sometimes they are widely separated (Pse- 

 laphognatha). In the male the legs of the first postcephalic 

 segment are modified (hooklike) in connexion with copulation. 

 On the seventh segment, and sometimes on the sixth and eighth 

 (in Glomeris on the penultimate segment) there is in the male a 

 complicated apparatus which may replace either one pair or 

 both pairs of legs and is known as the auxiliary copulatory 

 apparatus. It has been regarded as the modified legs of the 



