PROSOMATIC SEGMENTS OF LIMULUS 239 
in Stylonurus, but all five possess no special locomotor or prehensile 
power. According to Laurie this is a specially primitive form of the 
group. 
It is significant to notice from this sketch that with the absence 
of special prehensile terminations such as chele, or the absence of 
special locomotor functions such as walking or swimming, these 
appendages tend to dwindle and become insignificant, taking up the 
position of mere feelers round the mouth, and at the same time are 
concentrated and pressed closely together, so that their appendage- 
nerves must also be close together. 
This sketch therefore shows us that— 
Of the six foremost prosomatic appendages, the cheliceree and the 
four endognaths were, at the time when the vertebrates first appeared, 
in very many cases dwindling away ; the latter especially no longer 
functioned as locomotor appendages, but were becoming more and 
more mere palps or tentacles situated round the mouth, which could 
by no possibility afford any help to locomotion. 
On the contrary, the sixth pair of appendages—the ectognaths— 
remained powerful, being modified in many cases into large oar-like 
limbs by which the animal propelled itself through the water. 
It is a striking coincidence that those ancient fishes, Ptericthys 
and Bothriolepis, should have possessed a pair of large oar-like 
appendages. 
At this time, then, in strong contrast to the endognaths, the 
ectognaths, or sixth pair of appendages, remained strong and vigo- 
rous. What about the seventh pair, the chilaria of Limulus ? 
Of all the prosomatic appendages these are the most interesting 
from the point of view of my theory, for whereas in the scorpion of 
the present day they have dwindled away and left no trace except in 
the embryo, in the sea-scorpions of old, far from dwindling, they 
had developed and become a much more important organ than the 
chilaria of Limulus. 
In all these animals a peculiarly striking and unique structure is 
found in this region known by the name of the metastoma, or lip-plate 
(Figs. 8 and 104 (7)); it is universally considered to be formed by 
the fusion of the two chilarial appendages, 
All observers are agreed that this lip-plate was freely movable. 
Nieskowski considers that the movement of the metastoma was 
entirely in a vertical direction, whereby the cleft which is seen 
