442 



GEOLOGICAL SUEVEY OF THE TEREITOEIES. 



parison. The early stages of Mysis, as worked out by Van Benedeu 

 and Claparede, and of Nebalia, aremnch alike ; the formation of the blasto- 

 derm is much the same. The nauplius stage in the egg is nearly iden- 

 tical in both, but beyond this the parallelism ceases to be an exact one ; 

 Nebalia turns oft' and follows quite a different developmental path from 

 Mysis or any Decapod. If we compare the young of Nebalia, taken from 

 the brood-sac, with that of Mysis, as figured by Clajiarede (Plate XVII, 



rig. 68.— Embryo of Nebalia ready to hatch, enlarged; ant', 1st antennfe; ant", 2d antennae; «&./., 

 abdominal feet or uropoda. The first maxilla crosses the thoracic feet. After Metschnikoff. 



fig. 6), or a more advanced stage, particularly that of Pseudomma roseum, 

 as figured by Sars,^ we shall find that many of the differential char- 

 acters which, in the adult, separate the Phyllocarida from the Becapoda, 

 are to be found in the young. In Mysis and allies at the same stage as 

 Metschuikoff's, fig. 18 of Nebalia, (our Plate XXXIII, fig. 4,) the 2d 

 antennse are simple instead of being bifid as in Xebalia; there are no 

 maxillipedes, and the maxillae are, as in the adult, immediately suc- 

 ceeded by the eight pairs of thoracic feet; moreover there are no ab- 

 dominal feet in Mysis or Pseudomma, while three pairs are present in 

 the young Xebalia. But with the exception of the lack of abdominal 

 feet in the Mysidce at this stage, it may be thought upon the whole, as 

 has already been stated by Balfour, that " the development of Xebalia 

 is abbreviated, but from Metschnikoff's figures may be seen to resemble 

 closely that of Mysis. . . . There is in the egg a nauplius stage 

 with three [pairs of] appendages, and subsequently a stage with the 

 zoea appendages." It seems to us that the comparison ^ here made is, 

 as regards any resemblance to a zoea, loose and inexact, whether ap- 

 plied to the Mysidce or to the Phyllocarida. The stage of the Mysidce 

 succeeding the nauplius is characterized by the presence of the rudi- 

 ments of eight pairs of appendages, the two pairs of maxillae, and the 

 six pairs of thoracic feet of the Schizopodous type, while the zoea has 

 no thoracic feet at all, so that it would appear that the Schizopods do 

 not pass through a genuine zoea state like that of the higher Decapods. 

 Nor on the other hand is the Xebalia stage represented by Metschni- 

 kofi's fig. 18 (our fig. 4), a zoea stage, for the embryo has the rudiments 

 of eight pairs of thoracic feet, and besides those of three pairs of ab- 

 dominal feet, while there is a well-marked carapace and rostrum, as well 



iG. O. Sars, Mouog. over Mysider, Heft 1. Taf. IV, fig. 23. 



2 Glaus (Genealog. Gimdlage des Crust. Systems, p. 31), as we find since writing 

 tliK above, does not accept Metschnikoti's comparison of the young Nebalia with the 

 zoea, although he does not give the reasons for his dissent. 



