82 
'W. F. rUECELL. 
tlie Araclinids were derived in the first instance from 
tracheal books or from gill-books. 
I have endeavoured to demonstrate in a preceding para- 
graph that, since all the lung-saccules within the pulmonary- 
sac precisely resemble in their formation and structure the 
two oldest which appear outside of this sac, all the saccules 
must have had the same phylogenetic origin and must 
consequently all have originally been upon the posterior 
surface of the abdominal appendage. The question, there- 
fore, is whether the saccules of this primitive appendage in 
the ancestral Arachnid were tracheae or whether they were 
produced from sunken-in gill-lamellm. Whei'eas the appear- 
ance of a number of tracheae in such a position seems most 
improbable, the arguments in favour of tbe branchial origin 
appear overwhelming. Most important amongst these, next 
to the embryological evidence, is the undoubted general 
agreement and affinity between Limulns and Arachnida, 
first pointed out by Straus-Durckheim arid v. Beneden, and 
afterwards so ably demonstrated by Ray Lankester. The 
embryological side of the cpiestion and the probable manner 
in which the transition from gill-books to lung-books may 
have taken place has already been fully discussed (pp. 17-44) 
and need not be considered again. 
I shall only introduce here two figures of the abdominal 
appendages of Limulus for comparison with the pulmonary 
segment of a spider drawn in fig. 20. The appendages of the 
genital segment (text-fig. 6), which are homologous with 
those of the pulmonary segment, have no gill-books, but 
possess the pair of genital openings [cj. o.), tvliich would lie 
between the gill-books, if the latter were present. Text-fig 7 
represents a branchiate appendage, and it will be seen that if 
the gill-leaves were sunk into the appendage and the latter 
into the abdomen, we should have exactly the condition found 
in a spider (fig. 20). The large entapophyses {ec. t.) shown in 
the text-figures are not, however, homologous with those of 
the pulmonary segment {ec. t. 8, fig. 20). 
fi’he endeavour to derive all trachem in Arthropods from a 
