DEVELOPMENT OF LIMULUS POLYPHEMUS. I93 



Limulus first appears in the Jurassic, and the species differ but slightly firom those now 

 living. 



The more typical Phyllopoda made their appearance during the Triassic period. The 

 lowest group, however, the Estheriadse, appeared during the Devonian, a species referred 

 to Estheria being found in that formation in Europe. The Cladocera are not known to 

 have existed previous to the Tertiary period, and it was not until recently (1862) that Von 

 Hayden discovered the ephippium of a Daphnia in the Rheinish brown coal (Gerstaecker 

 in Bronn's Klassen und Ordnungen, etc.) said by Lyell to be of Eocene age. It should 

 be noticed, however, that the fossil belongs next to Sida, the most highly organized genus 

 of the group, and as it is not unlikely that such pelagic forms as Evadne may have existed 

 in the Mesozoic seas, if not earlier, I have ventured to run the point of the wedge into 

 the Carboniferous period.^ 



The Ajjusidte date back to the early part of the Mesozoic, a Triassic species of Apus 

 having been found in Europe, according to Mr. Salter. 



The soft bodied and most specialized group of Phyllopoda, the Branchipodidse, have not 

 been found fossil ; and though the conjecture that they are purely Quaternary, or at least 

 Pliocene forms, may be well founded, it would seem as if the question as to their geological 

 age would never be settled, unless impressions of their delicate bodies be preserved in 

 fine grained Tertiary shales. 



Our diagram may also be taken as expressing in an exceedingly crude manner the sup- 

 posed genealogy of the Entoniostraca, and the approximate periods when the types of the 

 orders, so far as we know them, were differentiated from the extinct hypothetical prototype 

 or ancestral form. Accepting the theory of evolution as a rational method of accounting 

 for the creation of the species and higher groups of animals, and the fact first brought out 

 by Fritz Midler, that the Crustacea as a rule pass through, either in the egg or larval condi- 

 tion, a nauplius state, and hence that all Crustacea have probably descended from a nauplius- 

 like ancestor, I believe that an attempt, however unsuccessful and apparently fatuitous, 

 to trace, from the few facts we now know, the probable ancestry of the different types of 

 the Crustacea, is logical and legitimate. At any rate the works of Fritz Muller, Anton 

 Dohrn, E. Haeckel and E. Van Beneden, are evidences that mere attempts at a geneal- 

 ogy of the Crustacea have greatly advanced their study, and opened us new fields of 

 research especially in their embryology and anatomy. 



Fritz Miiller has indicated, and so far as the nature of the case and our present knoAvl- 

 edge extends, demonstrated, that the majoi'ity of the decapodous Crustacea are born at 

 first as zoe£5e ; while one of the lowest genera, Penseus, is born as a nauplius, comparable with 

 the young of the Copepoda, Cirripedia, and certain Phyllopoda. Since the young of the 

 lowest Crustacea are born as nauplii, which are comparable with the nauplius of the Deca- 

 poda, and since many intermediate typical Crustacea have young of the same form, or pass 

 through a nauplius stage in embryonic life, it seems a reasonable theory of Midler's that 

 the ancestor of all the Crustacea was of the nauplius type. 



Miiller, however, passed over any consideration of the Trilobites and Merostomata ; and 

 were these and the group Nebaliadse disregarded, there would be no marked exceptions to 



1 Since the above was in type, I find in Giebel and ccra, Lyncites ornatus, fi-om the Carboniferous rocks of Saar- 

 Siewert's Zeitschrift (1870, vol. i, p. 524) a notice of tlie brucken I 

 discovery by Friedrich Goldenberg, of a species of Clado- 



MEMOIRS BOST. SOC. NAT. HIST. VOL, II. 49 



