SECT. II 



THE OUTER BODY FORM 



17 



five segments with no limbs developed, with no 

 ventral ganglia, and with no organs except the 

 most necessary, viz., the intestinal tube and the 

 musculature. Then follows a row of rudimentary 

 segments, each with a minute pair of limbs and a pair 

 of ganglia, which increase in size and development 

 from behind forward. The rudimentary segments 

 which have become fixed in the adult Apus do 



Tnicr- 



hp 



Fig. 3. — Lepidurus Spitzbergensis, from nature. The left half of the shield removed 

 to show the vermiform body. The first 14 trunk segments carry a pair of limbs 

 each, the following 10 "rings" carry between them ca. 28 rudimentary limbs, 

 and therefore correspond to 28 segments. The last 5 trunk segments (excluding 

 the anal segment) are limbless. bj>, brood pouch formed by the nth pair of 

 trunk limbs. 



not correspond with the rings of the body ; only 

 gradually as they recede from the limbless segments, 

 and thus are more fully developed, do the segments 

 correspond with the rings. The fusing of several 

 rudimentary segments to form one body ring, i.e. a 

 muscular segment, presents no difficulty. The Myria- 

 poda afford us several examples of the fusing of true 

 segments to form body rings. 



The study of the development of Apus has shown 



