No. 2.] THE EMBRYOLOGY OF LIMULUS. 2X\ 



of similarity is in the tendency toward a late appearance of 

 appendage I in both groups, it having been noticed by both 

 Metschnikoff and Laurie in the scorpion and by Birula ('92) in 

 Galeodes. This, however, has less weight than it otherwise 

 would have were it confined to these forms alone, Grobben 

 having noticed ('79) a similar delay in the appearance of the 

 antennulae in Moina. 



To several other points exceptions may be adduced. Thus a 

 chitinous entosternite has been noticed in several Crustacea, 

 e.g. in Apus and by Claus ('92) in Ostracodes. Deutova ^ occur 

 in both Arachnids and Limulus but as Zaddach described Ions: 

 ago ('4i) they are found in Apus as well. In the American 

 Limulus, as in the Arachnids, the coelom at first extends into 

 the appendages (Kishinouye says it does not in the Japanese 

 species) but similar conditions have lately been shown to occur 

 in the Hexapods. Reticulate genital ducts occur in the Phyllo- 

 pods. I cannot agree with Kishinouye that the metastoma of 

 Limulus possesses a separate somite, and as I have already 

 pointed out ('92, p. 6o) his figures can receive another interpre- 

 tation. There is no somite and no neuromere for the metastoma, 

 and as it occurs upon somite VI which is already provided with 

 appendages, its appendicular nature is not apparent. Meta- 

 stomal structures occur in other Arthropods; the exact serial 

 similarity between that of Limulus and that of the Arachnids 

 is the important point. 



The possession of a post-anal moveable spine (telson in Limu- 

 lus, sting in the scorpions, multiarticulate whip in Thelyphonus) 

 is not paralleled outside of these forms. It is to be regarded 

 not as a somite or a series of somites — the position of the 

 anus settles that — but as an articulated outgrowth of the supra- 

 anal region of the terminal somite of the body. 



The foregoing disposes of points 9, 13, 14, 15, 16, 17, 20, 25 

 and 26, while no discussion need here be given to items 8, 24, 



1 I have used this term, introduced by Claparede, for those molted cuticula or 

 " Blastodermhauten " which serve as protective envelopes — Packard's " vicarious 

 chorion " — after the splitting of the chorion, and before the young is turned free 

 to shift for itself. Henking has called ('82) the same structures in the Arachnida 

 "apoderma." 



