2i8 Alexander Petrunkevitcli, 



limit between the last thoracic and lirst al)dominal somite ( i. e. 

 between the 9th and loth somites). The reduction of the heart 

 always begins at the posterior end and proceeds forward. The 

 arterial system wherever fully developed consists of an aorta with 

 its arch and pedal arteries and a series of interneural arteries con- 

 necting the supraneural with the subneural artery. The legs are 

 typically of 7 joints the last of which in some cases may be sub- 

 articulated; the pedipalpi of 6 joints, the terminal joint corre- 

 sponding to the last two joints of the legs. The chelicerse are 2 

 or 3 jointed. The eyes whenever present are always simple, not 

 more than 12 in number and belong to the first and second somites. 

 External segmentation of the adult does not coincide with true 

 segmentation. 



To facilitate the understanding of the above definition the fol- 

 lowing table comparing apparent segmentation in several Orders 

 of Arachnida with true segmentation may not be without practical 

 value to students of extinct Arthropoda. 



The Table needs little explanation. The transverse lines indi- 

 cate the limits between segments as they actually appear to the eye. 

 not as derived from the presence of appendages or from the stud\- 

 of internal anatomy and embryology. The somitic value of the 

 sternum in the cephalothorax is omitted in all cases in order to 

 emphasize the differences in the composition of the carapace as 

 well as for lack of space. Attention is called to the fact that the 

 normal or most primitive type of segmentation is found not in 

 Scorpions but in Pedipalpi. There is still some difference of 

 opinion concerning the eyes in Araneae. Schimkewitsch and 

 Scheuring homologize the indirect eyes of spiders with the median 

 eyes of scorpions, and the direct eyes with the lateral eyes of scor- 

 pions. Schimkewitsch even gives a figure of the ganglia as 

 derived from the study of sections in embryos of Ischnocohis. It 

 seems to me however that the evidence adduced by the above men- 

 tioned investigators is not sufficient to decide the case in their 

 favor and I retain for the present the homology of the median eyes 

 of the scorpion with the direct eyes of spiders. 



It is not impossible that Arachnida possessed originally a few 

 somites in front of the eyes. Corresponding neuromeres have 

 been figured by Schimkewitsch. If such is the case the body of 

 an Arachnid is in reality composed of more than 21 somites, but 



