LIMULUS AN ARACHNID. 635 
terina in fact serve in a most important manner to directly 
confirm the assimilation of segments and appendages in the 
two animals which I have already insisted upon. 
In the first place, it is to be admitted once for all that 
Limulus and Scorpio agree with one another, and differ 
from the Eurypterina in possessing six pairs of cephalo- 
thoracic appendages. “An anterior pair has disappeared in 
the Eurypterina, and this reduction is the distinctive cha- 
racter of the order. That such a loss of an anterior pair of 
limbs has occurred is rendered probable by the fact that 
there is evidence of a tendency for this abortion of anterior 
appendages to go on further still. The actual anterior pair 
corresponding to the second pair of Limulus and Scorpio is 
very small in some Eurypterina (see fig. 20), and suggests the 
existence of causes tending to the suppression of appendages 
in the anterior region. Such a suppression of anterior ap- 
pendages is not without parallel among the Arthropoda (e.g. 
certain Crustacea), and for the Arachnida it has always been 
regarded as characteristic whenever the attempt has been 
made to compare the appendages of those forms with those 
of either the hexapod Insects or of the Crustacea. It is not, 
therefore, assuming too much when we admit that just as 
possibly (though I do not at the moment assert the fact) 
one pair of appendages is suppressed in all Arachnida as 
compared with other Arthropoda, so a second pair has been 
suppressed in the Eurypterine order of Arachnida. 
Counting the segments of the Eurypterina upon this 
assumption, we find that they exactly agree with those of 
the Scorpion. The segments succeeding the cephalothorax 
and anterior to the anus are twelve in number, gradually 
towards the anus, though not suddenly, diminishing in size 
after the seventh, as in Scorpio. Posteriorly to the anus 
is the postanal spine, broad and flat in most Eurypterina 
for swimming, and neither rod-like, as in Limulus, nor 
globose, as in Scorpio. Any difficulty which the unseg- 
mented telsonic region of Limulus may have presented in 
the comparison with Scorpio is removed by the simple 
inspection of the abdomen of the fossil Limuloid (woodcut, 
fig. 20). 
Secondly, a difference between Scorpio and Limulus of 
some importance is seen when the form of the cephalo- 
thoracic limbs is compared, since in Scorpio certain of 
those which are chelate, in Limulus are simple ambulatory 
organs. Here, too, the admittedly Limuloid Eurypterina 
remove all difficulty; for among them all the cephalo- 
thoracic appendages are in some genera non-chelate (fig. 20), 
