28 THORELL & LIND STRÖM, SILURIAN SCORPION PROM GOTLAND. 



and as in the Class of the Criistacea as well as in tJiat of the Arachnida, the number 

 of the segments of the body is so variable, the agreement of the Merostoms with the 

 Scorpions in this feature cannot be considered as of any great importance, more par- 

 ticularly as the body of Limulus cannot with certainty be shown to consist of more 

 than 15 segments, not of 19, as in the Scorpions and Eurypterids. The Copepoda etc. 

 have, just as the Merostoms and the Scorpions, a segment (telson) behind the anal 

 aperture, though it is not simple, as in the latter groups, being divided so as to form 

 a »furca». 



A greater classificatory importance must be attributed to the conformity, in the 

 animals in question, of the number (and in part also of the arrangement) of the appen- 

 dac/es of the cepkalothoracv, than to the similarity in the number of the body segments. 

 Both the Scorpions and the Merostoms have six pairs of cephalothoracic appendao-es, 

 with the exception of some Eurypterids, which have probably only five pairs. In the 

 Merostoms, however, all these appendages surround the mouth, and their coxte are all, 

 with the exception of the first pair, concerned in manducation, whilst in the Scorpions 

 only the coxaa of the second pair of appendages form true jaw-organs. The coxge of 

 the two following pairs have, it is true, in the Scorpions (and the Opilions) a process 

 directed forwards (or inwards), which also must be numbéred with the parts of the 

 mouth; but in all Arachnida, at least the two hindmost pairs of appendages are si- 

 tuated at in general considerable distances behind the mouth, and serve exclusively 

 as locomotive organs. Moreover it seems probable that the agreement in the number 

 of the appendages of the cephalothorax in Merostoms and Scorpions is more apparent 

 than real. The ^ilahruriui or »rostrum» of the Arachnida is originally^) a paired or- 

 gan, corresponding, according to Croneberg, to one or perhaps two pairs of cephalic 

 appendages, just as the antennas of the insects. In the embryo, in fact, it is at first 

 placed in front of the mandibles, but during the embryonal growth it gradually moves 

 backward, so that it comes at last to be situated, together with the oral aperture, 

 behind the mandibles^). The plate therefore which in Limulus is situated between 

 the oral aperture and the base of the tAvo foremost pairs of appendages, and which, 

 according to Ray Lankester himself, is a sternite, does not, as this author supposes, 

 correspond to the labrum of the arachnida. Also the identification of the metastoma 

 of the Euryptei-ids and the chilaria of Limulus with the sternum (metasternum) of the 

 scorpions must at once be abandoned, if we cast a glance at the arrangement of the 

 sternal parts in Eoscorpius euglyptus as described and figured by Peach^). 



') Compare A. Croneberg, Ueber die Mimdtheile der Arachniden, in the Archiv f. Naturgeschiclite, 1880, 

 p. 285 et sequ. 



^) As regards Scorpions and otlier Araohnids, we apply the term mandibles to their lirst pair of appen- 

 dages (the »cheliceree», »falces», etc.), and call the coxal joint of the second pair of appendages the maxilla, 

 the other joints of this pair forming together the palpus. These first and second pairs of appendages ori- 

 ginate in fact, in the embryo of the Araohnids (at least of the Spiders and Scorpions) from the first and 

 second post-oral segments respectively, just as the mandibles and the first pair of maxillae of the insects. 

 Also, the ganglion from which the uerves of the mandibles or chelicerae of the Arachnida originates, is, 

 according to Balfodr, at first post-oral, and separate from the cerebral or supra-oesophageal ganglion pro- 

 per. It is evident, therefore, that the first pair of appendages of the Arachnids, cannot possibly be identi- 

 fied with the antennae of the insects, which in the embryo belong to the prce-oral segment. 



^) On some new species of fossil Scorpions, etc, 1. c, p. 403, Pl. XXII, Fig. 3a. 



