LIMULUS AN ARACHNID. 5a 
bifid margin speaks of such an origin, and, as a matter of 
fact, such appears to be its embryological history. 
I shall here quote a passage from ‘ Balfour’s Embryology,’ 
recounting Metschnikoff’s observations upon the existence 
of rudiments of appendages in the segments of the Scorpion’s 
body following upon the cephalothorax with its six pairs of 
limbs. The observations have great importance, not only in 
reference to the genital operculum but also in regard to the 
pulmonary sacs and their ‘branchial books” which are 
found in succeeding segments. 
Balfour says, “‘ Rudimentary appendages appear on the 
six segments behind the ambulatory legs. . . . They persist 
only on the second segment, where they appear to form the 
comb-like organs or pectines. The last abdominal segment, 
i.e. that next the tail, is without provisional appendages. 
The embryonic tail is divided into six segments, including 
the telson. The lungs are formed by paired invaginations, 
the walls of which subsequently become plicated, on the 
four last segments, which bear rudimentary limbs, and 
simultaneously with the disappearance of the rudimentary 
limbs ” (‘ Comp. Embryology,’ vol. i, p. 359). 
Hence it appears that, in Scorpio, in front of the comb-like 
organs, that is to say, in the position subsequently occupied by 
the genital operculum, there is in the embryo, as in that of 
Limulus, a pair of rudimentary appendages. We know that in 
Limulus these grow together to form the genital operculum. 
It is in the very highest degree probable that the same history 
obtains for the similarly related genital operculum of Scorpio. 
In discussing the tergites, it has already been pointed out 
that the genital operculum corresponds to a separate band- 
like tergite in Scorpio (v11, in woodcut, fig. 2), and to an 
emarginated area on the anterior border of the abdominal 
carapace of Limulus (vir, in woodcut, fig. 1), which is more 
distinctly marked in the embryo. 
The eighth pair of appendages.—In Scorpio we find, on the 
ventral surface corresponding with the eighth tergite (six ter- 
gites being reckoned to the cephalothorax )a pair of appendages 
carrying fine lamellz set like the teeth of a comb along the in- 
ferior margin (woodcuts fig. 5 ga, left, and fig. 6 vi1t, left ; see 
also Plate XXVIII). They are developed from the second 
pair of rudimentary abdominal appendages of the embryo. 
In Limulus, in the corresponding position, we find a pair 
of appendages, the first of a series of five pairs (woodcuts fig. 5 
g@, right, and fig. 6 vit, right). The appendages of the two 
sides, as in the case of the genital operculum, do not diverge 
from one another but are directed towards one another and 
