LIMULUS AN ARACHNID. 507 
The Arachnida are then divided into three orders, “ les 
pulmonaires, les branchiféres, et les trachéens,” but it is not 
explained whether the term ‘‘ gnathopodes”’ is to be regarded 
as simply a synonym of the order “ branchiféres.” 
With regard to the internal sternum, the citation of the 
views of M. Straus runs as follows:—‘‘ Dans Vinterieur du 
thorax de tous les arachnides, a lexception peut-étre des 
acarides dont la plupart des espéces sont trop petites pour 
qu’on puisse les disséquer et connaitre leur organisation, on 
trouve une piéce cartilagineuse diversement configurée 
suivant les familles, et placée dans le thorax au-dessus du 
sternum. Cette piéce, a laquelle convient le nom de sternum 
interieur est maintenue librement par le moyen de plusieurs 
muscles qui se soudent de différents points de sa surface sur 
le bouclier, ou sur le sternum extérieur auquel ils se fixent. 
Elle sert en outre de point d’insertion 4 un certain nombre 
de muscles des pattes.” 
Since the time when Straus Durkheim put forward these 
views a mass of knowledge has accumulated which has 
tended to throw light on the affinities of Limulus. Of most 
importance has been the discovery of the complete form of 
the body of the paleozoic scorpion-like Arthropods known 
as the Eurypterina, and the quite recent (1873) thorough 
investigation of the nervous system of Limulus, by Alphonse 
Milne-Edwards, and further, the investigation of the de- 
velopmental history of Scorpio, by Metschnikoff, and of 
Limulus, by Dohrn and by Packard. The gradual growth 
of the recognition of the Arachnidan affinities of Limulus 
during the last twenty-five years is obvious enough, and yet 
all systematic writers, and all who have especially discussed 
the question, continue to classify Limulus among the Crus- 
tacea whilst speculating as to the possible derivation of 
the Arachnida from that form, or else place Limulus in a 
distinct group, neither Crustacean nor Arachnidan. 
I shall endeavour to show in the following pages that 
there is a much closer agreement of parts between Limulus 
and the Arachnida (especially Scorpio) than has been hitherto 
admitted by any one writer, even by Straus himself. 
It appears to me that the full extent of the agree- 
ment between Limulus and the Arachnida has never yet 
been stated, for whilst this or that observer has recognised 
one set of facts he has overlooked or misinterpreted another, 
and thus undervalued the indications of affinity between the 
two forms which he had admitted to exist. That the King 
Crab is as closely related to the Scorpion as is the Spider 
VOL, XXI, —NEW SER, LL 
