56 PBOCEEDINGS OF THE GEOLOGICAL SOCIEXT. [DeC. 20, 



spirit, he could not satisfactorily make out whether or not it has a 

 separate nerve-ganglion. 



He, however, concludes that " these two peculiarities make it im- 

 probable that Limulus belongs to the Crustacea." 



Dr. Dohrn remarks that " what Savigny has hinted at, and what 

 Strauss-Diirckheim has one-sidedly expressed, reappears now under 

 the light of the theory of evolution." The connexion of the Arach- 

 nida with the Crustacea is probably through Limulus and the Eury- 

 pterida, as indicated by myself in 1866 (see Quart. Journ, Geol. 

 Soc. vol. xxiii. loc. cit.). 



In comparing the Lhnuli and Eurypten with the Arachnida one 

 nattirally turns to the Scorpionidae as the group most suitable for 

 that purpose. 



Both in Pterygotus and Scorpio we find the antennary system 

 modified in the same manner, not only in form but in function also; the 

 larval ocelli are seen in both ; the locomotory appendages are all 

 cephalic in both; and the elongated body,with its peculiar trapezoidal 

 head-shield, its enlarged thoracic segments (the first in both) bearing 

 the reproductive organs on the underside, and the more slender 

 abdominal series followed by a broad apiculated tail-joint, naturally 

 seems to invite a comparison of the two groups. But the respiration 

 in the former is performed aerially by tracheal openings distributed 

 in pairs along the ventral borders of the thoracic segments ; whereas 

 in Limulus and Pterygotus the respiration is performed by branchiae 

 borne on the same series of segments (the thoracic) in Limulus, and 

 by one or two only of the same in Pterygotus. But, on the other 

 hand, the great cordiform under lip of Pterygotus and the two pieces 

 homologous therewith in Limulus, the broad thoracic plate or oper- 

 culum in both Limulus and Pterygotus, and also the large compound 

 eyes and the respiratory system in both, differ entirely from any thing 

 occurring in the Arachnida. I have long suspected that the organs 

 called combs in the Scorpion may be homologous with the branchial 

 leaves oi Pterygotus : they are, so far as we are aware, aborted organs 

 in Scorpio, although they have been supposed to fulfil some exci- 

 tatory function in connexion with the impregnation of the ovaries ; 

 but I can find no evidence upon the subject. 



As I have elsewhere stated, there is no insuperable difficulty in 

 accepting, on sound physiological grounds, the possibility of any 

 animal passing through larval conditions, casting aside at even a 

 single moult its branchiae, and assuming aerial respiration, quitting 

 the water and inhabiting the land, changing its element, its diet, its 

 mode of progression, and its entire life. Such cases are familiar to the 

 entomologist *, the carcinologist f, and even to the herpetologist +. 



From the examination of the embryonal changes of Limulus 

 Dr. Dohrn has already been able to prophecy much ; and no doubt 

 more will follow from his researches, if he obtains better specimens. 



He sees, however, how remarkably the young stages agree with 



* Larval and adult Libellulag, Ephemera, &c. 

 t Gecarcinus ruricola and other land-crabs. 

 1 The Batrachia. 



