524 MR MALCOLM LAURIE ON THE 



an arthropod which breathes by gills must be a crustacean, but as Lankester * and 

 Claus have pointed out in the case of Limulus, the morphological value of this fact has 

 been greatly overestimated. The branchia of Eurypterids, like those of Limulus, are 

 constructed on a type unknown among the Crustacea, and further, structures such as 

 these, which are the product of a physiological necessity, are not of much value as 

 indicating close relationship. Against the crustacean relationship must be put the segmen- 

 tation of the body and position of the genital aperture — which does not agree with that 

 of any known crustacean — the absence of anything representing the first antennae, the 

 chelate structure of the one pair of preoral appendages, and the fact that there is no trace 

 of the typical crustacean biramous structure in the appendages. The presence of compound 

 eyes has been urged as a resemblance, but the eyes were most probably constructed on 

 the same plan as those of Limulus, which have been shown + to be at all events very 

 different in type from those of the Crustacea. Further, the Crustacea have all — with the 

 exception of a few Ostracods — three pairs of appendages specially modified as mouth 

 organs, and modified in a more or less definite way as mandible and first and second 

 maxillae. Even in a low form like Apus, though all the thoracic appendages are to some 

 extent masticatory in function, nevertheless the first three pairs are very different from 

 the rest. Of this specialising of the first three pairs of postoral appendages, there is no 

 trace in the Eurypteridae ; and, indeed, instead of the chief masticatory function being 

 acquired by the first pair, it is always best developed in the last pair. 



On the whole, then, there seems very little reason for considering the Eurypterids as 

 related at all closely to the Crustacea. If their relationship to the Arachnida be admitted, 

 as I think it must, the Eurypterids may be considered as intermediate between Crustacea 

 and Arachnida, in the sense that they are among the most primitive Arachnids, and 

 therefore, nearer the junction point of the two stems ; but that they show any points of 

 affinity to the Crustacea beyond the fact that they are arthropods must be considered as at 

 all events not proven. That the point of union of the two stems was a very much simpler 

 and less specialised form, is very clearly indicated, especially if we regard the nauplius 

 larva of Crustacea as possessing any phylogenetic value, nothing at all comparable to it 

 having been found among other Arthropoda. 



Limulus. 



Whatever doubts there may be as to the relation of Eurypterids to other forms, there 

 is an almost universal consensus of opinion that they are closely related to Limulus. The 

 detailed comparison of the two forms has been so thoroughly worked out, that I need 

 not enter into it here. The only points of importance which I have been able to add to 

 the resemblance are the existence of preoral chelicerae and abdominal appendages. The 

 latter differ in Slimonia at all events from those of Limulus, chiefly in not being united 



* Quar. Jour. Micr. Sci., vol. xxi. t Lankester and Bourne, Quar. Jour. Micr. Sci., vol. xxiii. 



