No. I.] THE EMBRYOLOGY OF LIMULUS. 59 



cumulus and posterior cloud occur only in these forms ; and the 

 succeeding stages are almost equally close. As I correllate 

 them, my figures of Limulus are to be compared with those of 

 the true Arachnids as follows : — 



Limulus. Arachxida. 



Fig. 21 Agelena, Locy, Fig. i ; Kishenouye, Fig. 5. 



Fig. 23 Agelena, Kislienouye, Fig. 5 ; Balfour, Fig. i . 



Fig. 24 Agelena, Locy, Fig. 3 ; Scorpio, Metschnikoff, Pi. 



XVII, Fig.'2. 

 Fig. 25 Scorpio, MetsclinikoflF, PL XVII, Fig. 3 (one less 



somite) ; Laurie. Fig. 17 (one more segment 



and lacks primitive groove) . 

 Fig. 26 Agelena. Schimkewitsch, PI. XVIII, Fig. I ; Balfour, 



Fig. 3. Locy, Fig. 6; Scorpio, Metschnikoff, 



PI. XVII, Fig. 6. 



A slight comparison of these figures will show that previous 

 to the appearance of the limbs there are a remarkable series of 

 parallels. Limulus agrees with the Arachnids and differs 

 from the Crustacea in the external appearance and growth 

 of the germinal disc ; in the considerable development of 

 metamerism before the appearance of the appendages,^ and in 

 the simultaneous appearance of the anterior five or six pairs of 

 appendages. When one of the six is lacking at first, it is 

 apparently the anterior pair which forms later. This has been 

 shown by Balfour, Schimkewitsch, and Kishenouye in Agelena ; 

 by Metschnikoff and Laurie in Scorpio, and by Dohrn and 

 myself in the present paper. On the other hand, Claparede 

 ('68) describes the sixth pair as lacking in Myobia, and Van 

 Beneden ('51) gives the same account of Atax. Limulus agrees 

 with the Arachnids and differs from the Crustacea in the total 

 absence of a nauplius stage. 



August, 1891. 



POSTSCRIPT. 



Since the foregoing pages were in the printer's hands K. Kish- 

 enouye has published his complete paper on the development 



1 In Chelifer (Metschnikoft", '70), the chelicer.e apparently are formed before 

 the somites are outlined. 



