No. 2.] THE EMBRYOLOGY OF LIMULUS. 249 



nificance than those between Chilopods and Diplopods (Chilog- 

 naths). This was pointed out some years ago by Mr. Pocock 

 ('87), of the British Museum, while I, independently ('88), 

 stated similar conclusions. 



Until we know more of both the structure and the ontogeny 

 of the Myriapod forms the correctness of this view cannot be 

 regarded as settled, but in the present state of our knowledge 

 the following facts seem important : 



The Diplopod head bears, besides the antennae, but two pairs 

 of appendages, — a pair of mandibles and a lower lip, com- 

 posed of a pair of coalesced maxillae.^ In the Chilopod the 

 conditions are as in the Hexapod, two pairs of maxillae being 

 present. 



In the Chilopods as in the Hexapods, each somite bears a 

 single pair of appendages, while in the Diplopods the majority 

 of the segments bear two pairs of appendages, and the re- 

 searches of Heathcote show that each segment is in reality 

 composed of two coalesced somites, a condition without parallel 

 elsewhere in the Arthropoda. In the Chilopods there is a 

 wide sternum separating the coxae of the ambulatory append- 

 ages ; in the Diplopods the coxae are approximate, and the 

 sternum is exceedingly narrow, or even entirely absent. 



In the Chilopods the stigmata, a pair to a somite, are lateral 

 (dorsal in Scutigera), and are placed above and outside the 

 insertion of the limbs, exactly as in the Hexapods. The 

 tracheae which arise from them are branched, and the intima 

 is thrown into a well developed spiral thickening as in the six- 

 footed insects. In the Diplopoda, on the other hand, the stig- 

 mata are beneath or even in the coxas, while the tracheae 

 (except in the Glomeridas) are tufted and unbranched, and the 

 thickening of the intima is poorly developed. 



In the Diplopods there are well developed foramina repug- 

 natoria upon the sides of each somite of the body. Such 



1 The attempt made to show that this lower lip is a " gnathochilarium " com- 

 posed of the two coalesced lower jaws, or first and second maxillae of the Chilog- 

 naths receives no support from the embryology of Julus (Heathcote '88), where 

 there is but a single somite when the hypothesis calls for two. Further the inner- 

 vation of the sense organs of the lower lip {cf. vom Rath. '86, PI. XX, Fig. i) 

 shows that but a single pair of appendages is concerned in the part. 



