DEVELOPMENT OF LIMULUS POLYPHEMUS. I99 



follow that there are not causes in operation that result in the evolution of specific and 

 higher categories among organized beings. 



The appearance however, of the three highly developed types of Crustacea, in the prim- 

 ordial period, indicates quite plainly that these and probably all the orders and perhaps 

 suborders of this class had an independent origin, and have not diverged one from another 

 at different periods. If the Nebaliadse are a simple family of Phyllopoda, then we have 

 the case of a family form starting in the lowest zone of the Silurian and surviving until 

 the present day in undiminished vigor, and indeed in a rather more highly organized form 

 than at the start. K the Decapoda were not derived from some Silurian ancestor which 

 drew its origin independently from some Lawrentian nauplius, I do not see from our present 

 facts how they could have branched off from the Silurian Nebaliadre, as we have no inter- 

 mediate types ; but speculation is useless where further discoveries may any day reveal 

 connecting links. But according to our present knowledge the Decapoda and Tetra- 

 decapoda probably arose independently of the Branchiopoda, and the types became fixed 

 late in the Silurian. For these reasons I am unable to agree with the opinion of Dr. A. 

 Dohrn ^ that the "Decapoda, Stomatopoda, Edriopthalma, as well as the Cladocera, Ostra- 

 coda, Copepoda and Cirripedia have arisen from the Ply'llopoda." This is denying a common 

 nauplius ancestral form for the Crustacea. Moreover, Dr. Dohrn seems to have overlooked 

 the fact that well developed Ostracoda (Leperditia and Primitia, represented by eight spe- 

 cies), so far as the valves of the carapace indicate, flourished in the lowest Silurian zone 

 both in Europe and America, and the probabilities are that they had companions of their 

 own order at least. For this reason, if no other, we would also question whether as Dohrn 

 claims, the Ostracoda are degraded, retrograde Ph^dlopoda. 



Since this article has been in the hands of the printer I have received (Jan. 16, 1872) 

 from Dr. A. Dohrn his essay "Zur Erabryologie und Morphologic des Limulus Polyphemus" 

 (Jenaischen Zeitschrift, Band vi. Heft. 4) ; with the few exceptions noticed below I am 

 greatly pleased to find both Dr. Lockwood's and my own conclusions in the main confirmed 

 by this able investigator. Dr. Dohrn concludes that "Limulus is nearest related to the 

 Gigantostraca ; both appear to be related to the Trilobites, though this relationship cannot 

 be established in all the details. The morphological and genealogical relations of these 

 three families to the Crustacea are not such as to be surely determined, perhaps they will 

 remain always doubtful. That they are related to the Arachnida we are not as the matter 

 now stands in a position to allow. So it only remains for us to put these three groups 

 under a common name, for which I might adopt Haeckel's expression 'Gigantostraca' and let 

 them take their place in the system with (neben) the Crustacea." Whether I have gone 

 too far in enlarging the old group Branchiopoda to include the Merostomata and Trilobita, 

 I leave to zoologists to decide. I think that since embryology throws an uncertain light 

 upon the subject, the question will have to be decided by a study of the young and adult 

 stages ; and from the fact that Limulus and the Eurypterida have branchial feet, comparable 

 with those of the Phyllopoda (the fact that the lamellae on the second pair in Limulus are 

 arranged like the leaves of a book, while in Pterygotus they are arranged side by side like 

 the teeth of a rake, to which Dr. Dohrn compares them, shows that the number and char- 



^Geschichte des Krebsstammes, nach embryologischen, etc. JenaischeZeitscbrift, Bd. vi, Heft, i, 1870. 



