LIMULUS AN ARACENID. 645 
face of the anterior region of the body were simple eyes, 
but arranged in two lateral groups and a central group. 
From such a form the Xiphosura were derived by reten- 
tion of the full number of the appendages of the prosoma, 
the limitation of the segments of the mesosoma to six, and 
their specialisation as plate-like organs serving as genital 
operculum, branchiz and swimmerets, further by the limi- 
tation of the segments of the metasoma, first of all to six, 
‘ and their subsequent fusion and partial disappearance even 
from embryonic expression, and the atrophy of the append- 
ages proper to them. At the same time the lateral groups 
of simple eyes were replaced by a peculiar form of com- 
pound eye. 
The Kurypterina diverged from the Xiphosura after most 
of these features had been elaborated, but so as to retain the 
six free segments of the metasoma, whilst at the same time 
they lost one pair (probably the most anterior) of the 
appendages of the prosoma, and possibly the three hinder- 
most of the appendages of the mesosoma. 
The Trilobita diverged from the common ancestry of the 
Xiphosura and Kurypterina probably at a time when the 
number of six segments to the mesosoma and six to the 
metasoma had not become a definite limitation, and when 
appendages were carried by both those regions of the body, 
differing only from the leg-like gnathites of the prosoma in 
possessing a second ramus and lamelliform branchial pro- 
cesses. Possibly the compound eye of the Trilobite was 
inherited from an ancestor common to it and the Euryp- 
terina. According to Walcot (12), a very distinctive feature 
in the differentiation of the Trilobita was the reduction of 
the number of appendages of the prosoma from six pairs to 
four. In all these forms it is important to note that the 
appendages of the prosoma, whether six pairs in number or 
less, whether chelate, tactile, ambulatory, or natatory, so 
far as the ‘ palp’ or chief ramus is concerned, yet all, with 
the exception of the most anterior pair, continue by means 
of their enlarged basal joint to act as manducatory organs. 
As a set-off to the loss of the manducatory functions of their 
coxe, the first pair possess, with rare exceptions, nipping or 
stabbing palps. 
The relationship of the Scorpion and other living Arach- 
nida to the Merostomata appears to be this. From an 
ancestral form, which was nearly related to the common 
progenitor of the Xiphosura and Eurypterina, which pos- 
sessed six pairs of appendages to its prosoma, the terga 
united to form a carapace, six free segments to its meso- 
