TRANSACTIONS OF SECTION D. 633 



the first pair of appendages in the Eurypterida ; but it may be said that they have 

 since been observed in Eurypterus Jischeri of the Russian Silurian/^'* and E. scoticus 

 from thePentland Hills/ -''in both of v?hich they consist of small chelate appendages 

 flexed and limuloid in detail, somewhat reduced perhaps, and enclosed by the 

 bases of the succeeding limbs, which become apposed as the anterior end is reached. 

 Since by this discovery the Limuloids, Eurypterids, and Scorpionids are brought 

 into a numerical harmony of limb-bearing parts, we may at once proceed to 

 other points at issue. So far as the broader structural plan of Limulus and the 

 Scorpion are concerned, all will agree to a general community, except for the 

 organs of respiration ; but concerning the ccelom, the mobile spermatozoa, and the 

 more detailed features under which Limulus is held to differ from the Crustacea and 

 to resemble the Arachnida, I would remark that motile spermatozoa occur in 

 the Cirripedes and Ostracods,'"' and that the rest of the argument is weakened, by 

 the probability that the 'arachnidan' characters which remain may well have been 

 possessed by the crustacean ancestors, and that Limulus, though specialised , being still 

 an ancient form, might but have retained them. The difficulty does not seem to 

 me to lie in this, nor with the excretory organs, if we are justified in accepting the 

 aforementioned argument that the so-called Malpighian tubes may be inturiied 

 nephridia, ectodermal in origin, and in knowledge of the existence of endodermal 

 excretory diverticula in the Amphipods.'"' These facts would seem to suggest that 

 as our experience widens, differences of this kind will disappear. 



As to the tracheal system, now adequately recognised by the upholders of 

 the arachnid theory, the presumed origin of tracheae from lung-books, the 

 probability that the ram's-horn organ of the Chernetidse may be tracheal, >'- the 

 presence of tracheae in a simple form in the Acari,"'^ and, by way of an anomaly, in 

 a highly organised form on the tibiae of the walking legs of the harvestmen 

 (Phalaugidae),^" are all features to be borne in mind. AVhile I am prepared to 

 admit that this wide structural range and varied distribution of the tracheae lessens 

 their importance as a criterion of affinity, I cannot accept as conclusive the 

 evidence for the assumed homology between lung-books and gills."'^ And here it 

 may be remarked that a series of paired abdominal vesicles, recently found in the 

 remarkable arachnid Koenenia, invaginate as a rule but in one example everted, 

 seized upon in defence of this homology, have not been so regarded by one most 

 competent to judge. '^** 



There remains the ento.sternite, an organ upon which much emphasis has been 

 placed. Not only does a similar organ exist, apart from an endophragmal system, 

 in the Phyllopod Apus, in Cyclops, and some Decapods ; ^'^'' but, regarding the ques- 

 tion of its histology, it may be pointed out that from all that is at present known, 

 the structural differences between these several entosternites do not exceed those 

 between the cartilages of the Sepia body.i<=- And when it is found that the figures 

 and descriptions of the entosternite of Mygale (' Mygale sp.,' ' Mygalomorphoui 

 Spider,' auct.) have been twice presented upside down ! ^''^ the reliability of this 

 portion of the argument is lessened, to say the least. 



_ Eecent observation has sought to clench the homology of the four posterior 

 pairs of limbs of the King crab and Scorpion, by appeal to a furrow on the fourth 

 segment in the former, believed to denote an original division into two ; but I 

 hesitate to accept this until myological proof has been sought.^'" 



Eeturning, amidst so much that is problematic, to the sure ground of palaeon- 

 tology, I wish to point out that when all is considered in favour of the arachnid 

 theory there still remains another way of interpreting the facts. 



In both Limulus and the Scorpion the first six of the eighteen segments are 

 well known to be fused into a prosoma bearing the limbs, but vphile in the Scorpion 

 the remaining twelve are free, in Limulus they are united into a compact opistho- 

 fiomal mass. In dealing with the living arthropods, there is no character deter- 

 minative of position in the scale of this or that series more trustworthy than the 

 antero-posterior fusion of segments. It has been called the process of ' cephalisa- 

 tion,' and the degree of its backward extension furnishes ihe most reliable standard 

 of highness or lowness in a given assemblage of forms. In passing from the lower 

 to the higher Crustacea, we find this fusion increasing as we ascend : and it therefore 



