CIRCULATORY SYSTEM IN ARACHNIDA 179 



of the scorpions. The idea that Limulus is an arachnid as it 

 is usually expressed, or more correctly that the Arachnida have 

 a common ancestor with Xiphosura, must now be completely 

 abandoned. Geologically, Limulus is older than the scorpion 

 and already the oldest limuloid shows the same type of segmenta- 

 tion as the recent. Neither has the idea of Versluys on the origin 

 of gills from lung books any bearing upon the question of origin 

 of Limulus or Arachnida. With a stress on imagination one may 

 derive Limulus from a eurypteroid ancestor, but to derive the 

 latter from originally air-breathing Arachnida on the basis of no 

 other evidence than conjectures which rest on. a comparison of 

 gill-plates with lung books and in the absence of any remains 

 of air-breathing Arachnida antedating eurypteroids, seems to be 

 a rather hazardous undertaking. 



It may be interesting to mention in this connection that early 

 stages in the embryonic development of scorpions show clearly 

 eighteen postoral neuromeres, the first of which soon passes in 

 front of the mouth and represents the chehceral somite. The 

 study of preoral neuromeres in the same stages is too compli- 

 cated to admit of impartial judgment. In later stages, after 

 the passage of the mouth behind the cheliceral neuromere, one 

 may clearly count three pairs of nerves issuing from what appears 

 to be three corresponding neuromeres. The first pair are the 

 optic nerves of the median eyes, the second the nerves of the 

 lateral eyes, and the third the nerves of the upper lip. These 

 nerves are much finer than both optic nerves and can be traced 

 with certainty only in sections parallel to the plane of symmetry 

 (sagittal). The adult scorpion has therefore four preoral and 

 seventeen postoral neuromeres. Five of the latter belong to the 

 thorax and twelve to the abdomen. It happens that the abdomen 

 of the scorpion shows twelve tergites. Yet one should not 

 conclude from this coincidence of figures that each of the neuro- 

 meres mentioned belongs to a corresponding tergite. Nothing 

 of the kind. T have already mentioned that the comb receives 

 its nerves from the third abdominal neuromere, as may be easily 

 demonstrated on sagittal and frontal sections. In early stages 

 the neuromeres do not possess longitudinal connectives and 



