with Observations on the General Arrangement of the Articulata. 285 



the second mode also, the coalescence, or anchylosis of two approximated 

 normal segments, takes place almost at the period of their formation. But 

 the original distinctness of the two continues marked throughout the whole 

 life of the animal, so that each moveable segment of the body is formed of two 

 distinct normal subsegments. Each of these subsegments retains its pair of 

 legs, both pairs being equally developed. This is the condition of the body in 

 the lowest or luViform Chilognatha. 



In the lower forms of the Chilopoda, the Geophilidce, there is a progressive 

 change in the mode of development. This takes place in the ovum. The two 

 subsegments of which each moveable division of the body of the perfect animal 

 (fig. 10.) is composed, and which subsegments are at first equally developed, not 

 only become anchylosed together before the embryo bursts from the foetal cover- 

 ings, but the posterior (8) of the two (7, 8) exhibits a marked superiority of 

 size. This difference continues to increase at each change of tegument, after 

 the animal has left the ovum, until each anterior subsegment has scarcely more 

 than one-half the extent of the posterior. This difference is greatest on the 

 ventral surface (fig. 16.), where the sternal plate of the posterior subsegment 

 covers nearly the whole. Coincident with the beginning of this change and 

 union of each pair of subsegments in the ovum, only one pair of legs is deve- 

 loped to each compound segment, and these have their origin in the posterior 

 of the two subsegments. Notwithstanding this difference in the extent of their 

 development, the rudimentaiy portions of the anterior segment still exist in 

 the form of minute, partially detached plates at the front of the posterior seg- 

 ment, the dorsal arc being represented by a very short transverse portion. 



In the higher genera of Chilopoda, as in Scolopendra (figs. 1 to 4.) the number 

 of compound, moveable segments to the body is greatly reduced, and a further 

 union of the original subsegments has taken place. The upper surface of each 

 moveable segment in this genus is covered by a single plate (d, e), on the an- 

 terior part of which there is only a slight indication of the original existence 

 of the first subsegment, in the form of an elevated, narrow, transverse band. 

 But there are still some remains of this subsegment on the ventral surface 

 (figs. I and 2.), At the anterior margin of the large ventral plate of each seg- 

 ment, there are on each side of the median line three very short, minute, 

 closely approximated, horny plates, disposed transversely across the under 



2 p2 



