392 



CHILOPODA. 



Metschnikoffs observations have not thrown as much light on 

 the nature of the double segments as might have been hoped, 

 but it appears probable that they have not originated from a 

 fusion of two primitively distinct segments, but from a later 

 imperfect division of each of the primitive segments into two, 

 and the supply to each of the divisions of a primitive segment of 

 a complete set of organs. 



Chilopoda. Up to the present time the development of only 

 one type of Chilopoda, viz. that of Geophilus, has been worked 

 out. Most forms lay their eggs, but Scolopendra is viviparous. 



FlG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS. 



(After Metschnikoff.) 



A. Side-view of embryo at the stage when the segments are beginning to be formed. 



B. Later stage after the appendages have become established. 



at. antennae; an.i. proctodaeum. 



The segmentation appears to resemble that in the Chilognatha, 

 and at its close there is present a blastoderm surrounding a 

 central mass of yolk cells. A ventral thickening of the blasto- 

 derm is soon formed. It becomes divided into numerous seg- 

 ments, which continue to be formed successively from the 

 posterior unsegmented part. The antennae are the first append- 

 ages to appear, and are well developed when eighteen segments 

 have become visible (fig. 175 A). The post-oral appendages 

 are formed slightly later, and in order from before backwards. 

 As the embryo grows in length, and fresh segments continue to 

 be formed, the posterior part of it becomes bent over so as to 

 face the ventral surface of the anterior, and it acquires an 



