174 ALEXANDER PETRUNKEVITCH 



ship with the loss of segmentation in the abdomen and proceeds 

 in the same direction, that is, from the posterior end forward. 

 The changes in the neural. portion of the circulatory system do 

 not extend over the thoracic haemomeres because of the per- 

 manency of the thoracic appendages, but follow the changes in 

 the position of abdominal neuromeres. As the contraction of 

 the longitudinal connectives between neuromeres brings ab- 

 dominal neuromeres into the thorax, abdominal vertical arteries 

 are also shifted in position, while the complete disappearance of 

 the last abdominal neuromeres brought about a corresponding 

 complete disappearance of the last vertical arteries. 



COMPARISON WITH LIMULUS 



The circulatory system of Limulus has been excellently de- 

 scribed by Milne-Edwards, and such errors as he has admitted 

 in his description have been later corrected by Patten and Reden- 

 baugh. I have made injections of adult large specimens to 

 verify the results, and can only confirm their correctness. It 

 is different, however, with the inteipretation of the structures, and 

 here I disagree both with the older and later investigators. 



Alphonse INIilne-Edwards worked eight years before Lankester, 

 and although the idea that Limulus is an Arachnid had been 

 already advanced by Latreille and later by Owen, yet the knowl- 

 edge was not sufficient to admit of incontrovertible homologies. 

 Consequently, notwithstanding the great similarity in the struc- 

 ture of the nervous and circulator}^ systems, Milne-Edwards 

 felt justified in pointing out the differences and in refusing to 

 place Limulus either among Crustacea or among arachnids. 

 For reasons which it is not worth while reviewing at present, 

 Milne-Edwards considered the first pair of appendages in Limulus 

 homologous, not with the chelicera, but of the pedipalpi in 

 scorpions. 



Lankester's interpretation of Limulus was colored by his 

 theory of tagmata into which (according to him) the body of 

 an arthropod is divided. He finds that the body of Arachnida 

 is composed of three tagmata of six somites each and that the 

 genital openings are placed on the first somite of the second 



