182 ALEXANDER PETRUNKEVITCH 



the rostral. I think we may consider it as fairly conclusive 

 that the median ocular and lateral ocular somites are homologous 

 in I jmulus and the scorpion. Whether the rostral somite of the 

 scorpion corresponds with the somite designated as rostral in 

 Limulus by Carpenter, is not so sure, but if it does not, then 

 Limulus must possess just the same some evanescent somite be- 

 tween the lateral ocular and cheliceral. As for the olfactory 

 somite, its homologue in Arachnida would have to be sought in 

 one of those two pairs of obscure parietal ganglia described 

 by Schimkewitsch for tetraneumonous spiders. 



In the presence of a perineural circulatory system, in the ex- 

 istence of eight pairs of ostia in the heart, and of a pair of chila- 

 rial nerves, Limulus shows evidence of its origin from an arthro- 

 pod ancestor lower and more primitive than the Arachnida. But 

 in every other respect Limulus shows advanced development 

 different from that in Arachnida and most likely standing in direct 

 relationship to its particular mode of life. It seems as if the 

 older interpretation of Limulus as a descendant of Trilobites 

 must be revived. The shape of the trilobite carapace, the posi- 

 tion of the mouth, the probable similarity in the position of the 

 foregut as suggested in figure 24 of Raymond's beautiful mono- 

 graph, the larval stages showing segments which were inter- 

 preted as cephalothoracic, but some of which probably are ab- 

 dominal tergites drawn into and fused with the thoracic ones, 

 point to a similarity more than casual. At any rate, the problem 

 should be reinvestigated from the new point of veiw. 



COMPARISON WITH OTHER ARTHROPODA 



We have seen that the aortic valve has a uniform structure 

 and a permanent position in Arachnida, permitting of strict 

 homologies within that class. We have also seen to what con- 

 clusions we arrive through the assumption that the rule holds 

 good in the case of Limulus also. One would expect that a struc- 

 ture so permanent in one or perhaps two classes would prove to 

 be the same in the case of all other Arthropoda, if the diverse 

 forms united under this immense phylum are of monophyletic 

 origin. Unfortunately, this is not the case. 



