194 A. S. PACKARD, JR., ON THE 



his beautiful theory that the young of all Crustacea are at first nauplii of some form, and 

 that the ancestral form was a nauplius.^ But on inspection of the diagram, it will be seen 

 that we have highly organized Trilobites and Nebaliadee existing in the beds of the 

 primordial zone, the lowest platform of life, where the oldest remains of animals higher 

 than the Eozobn are found ; while the Merostomata (Eurypterus) suddenly appear in the 

 Middle Silurian, the individuals found indicating as high an organization as the Devonian 

 forms, and we have no hint of any lower forms like them in the rocks below. Thus so far 

 as the facts known at the time of writing may be relied upon, crustacean life exhibiting 

 three very distinct types of structure appeared suddenly upon the surface of the globe. 

 We find no foi'ms peculiarly embryonic (and by embryonic I mean forms resembling the 

 embryos of other animals) among them. Associated with the highly organized Paradox- 

 ides, with its numerous thoracic segments, indicating a long series of moults and probably 

 many years of growth after passing through the larval condition, we find the truly larva- 

 like Agnostus, differing only from its own larva by the addition, according to Barrande, of 

 two thoracic segments acquired evidently by one or two subsequent moults. Indeed we 

 have, it would seem, no clue to the hypothetical, primitive, nauplius form. Thus we have 

 in the Trilobites, Merostomata and Nebaliadse, important exceptions to the mode of devel- 

 opment of the individual and of the order in other Crustacea, as we here have Arthropoda 

 which have already acquired a habit of metamorphosis, the embryo maturing into the larva 

 before hatching. We have here, then, an apparent contradiction ( 1 ) to the law, in the broadest 

 sense true, that the oldest animals are lower than those now living; (2) to the supposition 

 that the Crustacea may all be traced to a nauplius form, and (3) to the theory that the 

 type has been evolved by secondary laws of creation from an ancestral form, rather than 

 called into existence by a sudden, special, creative act. 



In point of fact, however, there are certain exceptions to the first two apparent contra- 

 dictions, wliich may be briefly indicated. First, among the Trilobites Agnostus is the 

 lowest form known, and approaches nearest the larvte of other Trilobites. But neither 

 this genus nor any other larval forms are known to exist above the Lower SUurian. Sec- 

 ondly, among the Branchiopoda (in the extended sense used by us) the lowest forms are 

 often among the largest, though this is a rule which must be used with caution. The 

 Nebaliadse, as remarked above, were gigantic in size in Paleozoic times compared with the 

 living species, and the species of the primordial zone are unquestionabl}' lower than the 

 living species. 



Turning to the Merostomata, we find the Middle and Upper Silurian forms, Eurypterus 

 and Pterygotus, to have attained in some species the almost incredible length of seven feet. 

 These are mature forms in distinctions fi-om larval forms, the species closely resembling the 

 most highly organized species of the Devonian and Cai'boniferous age. The latest species 

 (of Lower Carboniferous age) appear to be, so far as known (see Meek and Worthen's 

 Anthraconectes, nearly a foot in length probably), of much more moderate size. The 

 earliest Xiphosura, Bellinurus and allies (and possibly Cyclus), were much smaller than 

 their more highly developed modern representative, the king crab, which sometimes 



' " I have not mentioned the Pycnogonidse, because I do Barrande of the development of the latter. According to 



not regard them as Crustacea; nor the Xiphosura and Tril- Mr. Spence Bate, 'the young of Trilobites are of the 



obites, because, having never investigated them myself, I nauplius-form,'" p. 96. It should be borne in mind that 



knew too little about them, and especially because I am " Fiir Darwin" was written by the author while living at 



unacquainted with the details of the explanations given by Desterro Island, on the coast of Brazil. 



