No. 2.] THE EMBRYOLOGY OF LIMULUS. 237 



of the Hexapod, and — if we be permitted to recognize a 

 metastomal somite in the Crustacea — with the "head" of the 

 Tetradecapods and also of that of the Decapods as Hmited by- 

 Milne Edwards, but not with the Decapod head as understood 

 by Huxley. The argument which Huxley draws for placing 

 the division between head and thorax in the Crustacea between 

 appendages V and VI, is largely based upon his views of the 

 cervical suture of the crayfish which Dana long before ('5l) 

 showed to be untenable and which Ayers ('85) has more lately 

 reviewed. 



Upon the standpoint we have taken the cephalothorax of 

 the Arachnids and Limulus must be regarded as equivalent 

 to the combined head and thorax of the Hexapod. In the 

 forms first mentioned we find no tendency towards a differ- 

 entiation of this region except in the case of the Solpugids, 

 a knowledge of whose embryology would prove so interesting. 

 As the Solpugids are not primitive forms, and as no such 

 regional divisions occur in the more ancestral types, we would 

 rather suspect that the apparent existence of the Hexapod 

 thorax in the group was secondary and adaptional rather than 

 derived from a common ancestor. Thorell's view that the 

 Solpugids are Hexapods is not tenable. 



In this connection the multiarticulate character of the an- 

 terior appendages of both Limulus and the Arachnids is inter- 

 esting. In the Hexapods and Myriapods the mandibles are 

 at no time of either embryonic or adult life multiarticulate, 

 a fact which would apparently indicate that this appendage 

 had obtained its present form and function at an early period. 

 It is, as Lang has suggested, hardly to be supposed that the 

 well segmented corresponding appendage of the Arachnids 

 has been derived from the specialized mandible of the Hexa- 

 pods. The modification of the basal joints (coxa) of several 

 appendages in both Limulus and scorpions for manducatory 

 purposes should be alluded to here as well as the persistence 

 of the same number — six — of articles in the legs of these 

 animals. 



It would hardly seem necessary to review in detail the 

 arguments for the homology of the respiratory organs of 



