KONGL. SV. VET. AKADEMIENS HANDLINGAK. BAND. 21. N:0 9. 27 



V. ARE THE MEROSTOMS ARACIINIDS? 



As the shape of the legs of PalaBophonus may possibly be considered to support 

 the view, adopted by E. van Beneden, Ray Lankester, Mac Leod, Peach and others, 

 that the Scorpions are nearly related to the Merostoms (Gigantostraca) and that the 

 latter, along with the Trilohites, belong to the Glass of the Ärachnida, it is perhaps 

 necessary in a few words to indicate our position in regard to these debatable and 

 difficult problems, and to adduce the grounds, which prevent us from accepting the 

 above stated opinion, which has been maintained with great ability particularly by 

 Ray Lankester^). — 



It cannot be denied, that in several points there is a very great resemblance bet- 

 ween the Merostoms and the Scorpions, and it is consequently evident that the Ärach- 

 nida are more closely related to the Crustacea than are the Insects and the Myriopoda. 

 Already in the embryos of Limulns and the Scorpions, we raeet with this agreement, 

 though only during the first stages of their development: the embryo of Limulus soon 

 presents an appearance which closely reminds one of the Trilobites; and whilst the 

 embryo of the Scorpions löses its abdominal legs at an early period, these legs in the 

 Merostoms develop into the »operculum» and the lameliar, branchiferous abdominal ap- 

 pendages of these animals. If, as seems probable, the pectoral combs of the Scorpions 

 are derived from the second pair of the abdominal legs of the embryo, and if their 

 genital plate originates from the first pair of these legs — a suggestion, however, for 

 which there is no evidence whatever — then the genital plate and the combs of the 

 scorpions would indeed be, as regards their origin, identical with the operculum and 

 the first pair of abdominal appendages of the Merostoms. But the genital plate of the 

 scorpions is probably nothing more than the »sternite» of the first abdominal segment; 

 and as in many Ärachnida, for instance the Chelonethi, and in the Acari, the embryo 

 differs much from that of the Scorpions and the Spiders, we probably may not be en- 

 titled to draw, from the greater or lesser agreement between their embryos, any de- 

 finite conclusions as to the affinities between the animals in question. 



The resemblance between the Merostoms — or at least the fossil Eurypterids — 

 and the Scorpions in the number of the segments, and also often in the general form 

 of the body, is surprisingly great. In Eurypterus obesus Woodw. and E. scorpioides 

 Id., for instance, the posterior part of the body is suddenly narrowed into a many- 

 jointed tail, just as in the case of the scorpions. But in this respect a great number 

 of undisputed Crustacea, for instance most of the Copepoda, agree with the Scorpions; 



■*) See chiefly liis paper "Limuhis an Aracliuid», in the .Jonrn. of the Roy. Microscop. Soe., New Series, 

 XXI (1881). 



