REGIONS OF THE BODY. 



775 



FIG. 505. Embryo of a Scorpion (after 

 E. Metschnikoff). Kf Chelicerse ; Kt 

 pedipalpi ; B' to B' 1 ', the four pairs 

 of thoracic legs. There are rudi- 

 mentary limbs on the abdomen, but 

 the pregenital segment is not shown. 



usually in a typical form three of these. Of these the prosoma 

 or cephalothorax comprises all the segments in front of the 

 segment bearing the genital pore. The region of body (abdomen) 

 behind this sometimes falls into two parts, the mesosoma and 

 the metasoma, but the position 

 of the division between these two 

 parts is not constant. These i.e. 

 the mesosoma and the metasoma 

 form the opisthosoma or the ab- 

 domen of authors. As stated in 

 Chapter IV 7 (p. 323) * there are 

 in the embryos of spiders in- 

 dications of a pair of somites 

 in front of the cheliceral somites, 

 and in this region vestigial an- 

 tennae have been found in a 

 spider (Lycosa sp.).f Between 

 the prosoma and mesosoma em- 

 bryology has shown that another 

 segment exists for a time but dis- 

 appears. It is impossible to say with certainty whether this 

 pregenital segment belongs to the prosoma or to the mesosoma, 

 but here we have reckoned it as belonging to the prosoma. 

 Counting the pre-cheliceral embryonic somite found in spiders, we 

 thus have the following scheme of segments in Scorpio or 

 Limulus named after the appendages they bear : 



1 . Eyes or prechelicerae (see above) . Preoral 



2. Chelicerae. Preoral 



3. Pedipalpi. Paroral 



4. 1st pair walking legs. Postoral 



5. 2nd 



6. 3rd 



7. 4th 



8. Chilaria or Pregenital segment|| 



* G. H. Carpenter, Quart. J. Mic. Sci., xlix, 1906, p. 469. 



t Javovsky, Zool. Anz., xiv, 1891-2. 



+ According to the view embodied in the table on p. 325 the chelicerae 

 is reckoned as belonging to the second segment. In Chapter IV the eyes 

 are not regarded as appendages 



The remaining segments are of course postoral. 



The appendages of this segment disappear in all Arachnids except 

 in the Merostomata and Pycnogonida. 



Prosoma. 



