98 



MISCELLANEOUS. 



On the Systematic Position of the King Crabs and Trilobites. 

 By M. E. TAN Beneden. 



Whilst recognizing in theory that a classification founded upon a 

 single character cannot be a natural classification, and that it is not 

 by a single character that truly natural groups are distinguished, a 

 great many naturalists, and even eminent men, have departed in 

 practice from this principle, which is unanimouslj^ accepted in theory. 

 Thus in the system of classification of Latreille and Milne-Edwards, 

 almost unanimously accepted by entomologists, the Arthropoda are 

 divided into two great groups, in accordance with the characters of 

 their respiratory apparatus, and the Crustacea are distinguished from 

 all the other Condylopoda by their branchiae. Every Arthropod 

 with branchiae is a Crustacean ; every Arthropod with tracheary 

 respiration is either an Insect, a Myriopod, or an Arachnidan. 



By thus basing a classification upon the existence of a single cha- 

 racter, it becomes exceedingly easy and simple to decide the place 

 that such or such an animal should occupy in the classification. The 

 place of the Liniuli, for example, cannot be doubtful for a moment ; 

 the Xiphosura form, with Milne-Edwards and the great majority of 

 naturalists, a division of the class Crustacea, sometimes placed in the 

 group of the Branchiopoda by the side of the Phyllopoda and Trilo- 

 bites, sometimes approximated to the Isopoda ; whilst sometimes the 

 Poecilopoda have formed a separate division in the class Crustacea. 



But now that it is generally admitted that classification should 

 represent the true affinities of creatures (that is to say, their genea- 

 logical connexions), it is necessary to take|into account as much as 

 possible characters derived from the totality of their organization, 

 from the history of their palseontological development, and especially 

 from the history of their ontogenic or embryonic development, 

 which represents an abridgment of the history of their genealogical 

 development. 



I am indebted to the kindness of my learned friend, Dr. Packard, 

 of Salem, Massachusetts, for having been able to study here in Bel- 

 gium the whole embryonic development of Limulus Polypheinus. Dr. 

 Packard had the extreme complaisance to send me several portions 

 of ova and embryos of Limulus, deposited and fecundated upon the 

 American coasts ; and I have been able to follow all the phases of the 

 development of these singular creatures, whose affinities have been 

 completely misunderstood hitherto. Strauss-Diirckheim alone, found- 

 ing his opinion upon important anatomical characters, opposed the 

 current of received ideas with regard to the position of the LimuK, 

 and he put forward the opinion that the Gnathopoda should form a 

 separate order of the class Arachuida. 



The study of the embryonic development of these animals, and of 

 their anatomical characters, has led me to the following conclusions, 

 which I may now formulate : — 



I. The Limuli are not Crustacea ; they have nothing in common 



