180 ALEXANDER PETRUNKEYITCH 



are recognizable without difficulty only because of the constric- 

 tion between adjoining neuromeres. The last neuromere is 

 clearly situated in the last abdominal segment, and only later 

 moves forward and fuses partially with the penultimate neuro- 

 mere. There is, therefore, in the adult scorpion an abdominal 

 segment in excess of neuromeres. From an examination of nu- 

 merous series I have no doubt that it is the first postabdominal 

 or caudal segment and which therefore may have the value not 

 of a true somite, but of an anterior subdivision or segment of the 

 same somite to which the second postcaudal segment also belongs. 

 Here, then, son^ething happened the reverse of the fusion of 

 sclerites in the first two abdominal somites, namely, the sub- 

 division of the sclerite ring of a single somite into two distinct 

 scierite rings or segments, without a corresponding subdivision 

 of other structures in the same somite. 



It may be objected that such formation of pseudo-segments 

 has not as yet been described, either for Arachnida or other 

 Arthropoda, and that it were simpler to accept that the neu- 

 romeres really correspond to the visible segments, but in moving 

 forward lost connection with them and began to furnish nerves 

 to the next following. In other words, that the first abdominal 

 neuromere originally furnished the nerves for the genital opercula, 

 lost connection with the latter, and ceded this morphological and 

 physiological function to the second neuromere; that the same 

 happened to the second neuromere in relation to the comb, which 

 now received its nerves from the third neuromere. But this 

 explanation, besides being more complicated, suffers from another 

 weakness. The roots of nerves follow the displacements of their 

 neuromeres, but the nerves themselves obtain their connection 

 with the original appendages, even if some branch of the nerve 

 passes to another somite. This may be seen in Limulus and in 

 many other arthropods. But in the case of the first abdoniinal 

 neuromere of the scorpion there is not even a considerable or 

 appreciable displacement forward, so that there would be no 

 morphological reason of any kind for a loss of connection with the 

 genital opercula if these belonged to the first neuromere. 



