434 R. r. POCOCK. 



teenth, eigliteentli, and twentieth somites, and Craterostig- 

 mus with these organs restricted to the somites represented 

 by the fourth, seventh, twelfth, fifteenth, eighteenth, and 

 twentieth tergal plates, have descended along divergent lines 

 of evolution from a primitive Scolopendromorph with stigmata 

 upon all the leg-bearing somites except the last.^ But since 

 the fourth, seventli, twelfth, fifteenth, eighteenth, and twen- 

 tieth tergal plates in Craterostigmus represent the dorsal 

 elements of the third, fifth, eighth, tenth, twelfth, and four- 

 teenth somites of a Lithobiomorph, the similarity between 

 the latter and a typical Scolopendroid, with respect to the 

 location of stigmata, may be regai'ded as merely coincidental 

 without any infringement of probability. 



So, too, with regard to the resemblance in alternation of 

 larger and smaller somites. This alternation is no doubt 

 beneficial in the way of favouring flexibility and rapidity of 

 torsion. Its acquirement, therefore, may be regarded as 

 advantageous. But this does not explain why there should 

 be an exact correspondence in this particular, if the two 

 forms under consideration have been independently evolved 

 from a type in which all the somites were major. Why, it 

 may be asked, are the second, fourth, sixth, ninth, eleventh, 

 thirteenth, and fifteenth somites minor in their constitution 

 in both cases, and the others major ? The explanation is to 

 be found in the situation of the stigmata. The invariable 

 absence of stigmata from minor somites in the Chilopoda 

 justifies the view that the presence of these apertures and of 

 the tracheal trunks that arise from them is incompatible with 

 somites of that nature. This is supported by what occurs 

 in the Greophilomorpha, where a large number of breathing 

 orifices appears necessary for respiration underground, and 

 where all the somites are major and subequal. But functional 

 representatives of the minor somites of the Scolopendroid 



1 Tlie atrophy of the breathing orifice upon the first leg-bearing somite in 

 the Geopliilomorpha, the Scolopendromorplia, and Craterostigmus is no 

 doubt traceable to tlie modification impressed upon this somite by the great 

 development of the muscularity of the toxicognatiis. 



