LIMULUS AN ARACHNID. 515 
that seen in the Scorpion, but in essential points there is 
remarkable agreement. In both the carapace carries two 
paired groups of eyes. Nearer the middle line is a single pair 
of simple eyes (0c’), which in Scorpio have an almost central 
position; more laterally placed (quite laterally in Scorpio) 
is a group, on either side, of simple eyes (0c), which in 
Limulus are so closely aggregated as to form what is often 
called. “ a compound eye.” The compound eyes of Limulus 
have, however, been shown by Grenacher (3) to differ very 
much in structure from the compound eyes of either Crus- 
tacea or Insects, to which they have usually been asssimi- 
lated. They are more correctly interpreted—as the com- 
parison with Scorpio would suggest—as an aggregation of 
simple eyes. Such an aggregation (varying, according to 
the genus, in number from two to five) we find in a less 
compact form than in Limulus on the right and left side of 
the Scorpion’s cephalothoracic tergite. 
In both Limulus and Scorpio the cephalothoracie tergite 
covers in an area corresponding to the six leg-like appen- 
dages which are present in both animals, and may therefore 
be considered as representing six coalesced tergites (1 to V1). 
In Limulus the genital operculum which follows upon the 
legs, and also the metathoracic sternites or chilaria which lie 
between it and the bases of the last pair of legs, have been 
by some morphologists regarded as also indicating segments 
which should be reckoned to the cephalothorax, and accord- 
ingly eight coalesced tergites have been supposed to con- 
stitute the carapace of the King Crab, whilst only six can be 
reckoned for the Scorpion. In reality, however, the chilaria 
are not appendages at all, as is proved by their late appear- 
ance in development (Packard, 8) and their form; they are 
simply sternites corresponding to the pentagonal sternite 
placed between the bases of the last pair of legs in Scorpio 
(woodcut, fig. 5). As to the genital operculum of Limulus, 
though in the adult it is in some measure adherent to the re- 
gion of the cephalothorax, yet it has a tergal area correspond- 
ing to it in the abdominal carapace, and in the embryonic 
Limulus is clearly seen to belong to that region, and not to 
the cephalothorax. The innervation of the genital operculum 
from the esophageal nerve-collar has, as already pointed, 
out, no weight as an argument in favour of the association of 
that coalesced pair of appendages with the cephalothorax, 
for on the very same grounds it would be necessary to asso- 
ciate a large part of the middle region of the Scorpion’s 
body (as far as and inclusive of the second pair of pulmonary 
sacs) with the cephalothorax. 
